I just sent an email to evga support for this exact issue. Used a macro editor, scroll up is doing up/down/down and scroll down is doing down/up/up. Hoping for an email back from them with news.

I am having this problem as well, has anyone heard any news on it?

Sooooooooooooo, I may have found a solution. Basically, do your best to stress the wheel. In my case, I unplugged the mouse and rolled it on my desk for a solid minute with a little pressure. The issue still happens but much much much less frequently. I got the idea on a YouTube video comment section stating the following:

"I called evga today.. Talked with support. What they have you do, and it worked for my mouse, is disconnect the mouse, turn it upside down and roll it from the bottom of your palm to the middle of your longest finger. Apply medium pressure and so the mouse rolls back an forward across the hand and finger for about ten seconds. I had to do it a couple of times. But my mouse is working fine now. It might just be really tight tuning. It needs to be broken in."

The_King_of_Normies
Sooooooooooooo, I may have found a solution. Basically, do your best to stress the wheel. In my case, I unplugged the mouse and rolled it on my desk for a solid minute with a little pressure. The issue still happens but much much much less frequently. I got the idea on a YouTube video comment section stating the following:

"I called evga today.. Talked with support. What they have you do, and it worked for my mouse, is disconnect the mouse, turn it upside down and roll it from the bottom of your palm to the middle of your longest finger. Apply medium pressure and so the mouse rolls back an forward across the hand and finger for about ten seconds. I had to do it a couple of times. But my mouse is working fine now. It might just be really tight tuning. It needs to be broken in."

Rolled the mouse back and forth on my hand for a bit. Seems to have fixed it!  Thanks!

Wow this actually worked. I got this back in December and shortly after this issue started. Info I found then was try cleaning it. It did make it okay for awhile. Then came back on an off a bit it but was back to unbearable now. When regular cleaning attempts didn't work I took it apart and cleaned out anything I could find in the wheel area. It didn't help at all. I decided to try finding help online again before giving up on it.

So I turned it upside down and ran it up an down firmly on the big mouse pad on my desk. I figured nothing would happen but it's fixed. I have no idea why that would work when actual cleaning didn't.

EvilPickle
Felt I should update my post..  running the scroll wheel over my pad or hand seems to help for a bit, but it starts messing up again soon after.  I've had to go back to my old mouse as the scrolling is just un-usable.  Add in the constant disconnects and I've just given up on this mouse. 

From I what I've experienced, if the issue happens to return (say after 24 hours), you just haven't rolled it enough. I had to roll mine for about 10 minutes with a bit of pressure to completely fix it. It's been a month and I haven't had this issue at all. Make sure your also doing long strides (3 feetish) with firm pressure. Also changing the direction I rolled it (in my case just up and never reversing) helped. 

OK update for my Mouse I called EVGA Support and they told me known issue so they accepted to do and RMA. so got my new X15 working great now. the 31 may. You Gent probably need do RMA as well

The cause seems to be from a bad scroll wheel encoder; unfortunately I asked EVGA support about what model of encoder was used and they do not know.  They claim it's a standard encoder, but only 1 result on google will show a similar looking encoder as the one used in the x15.  
 
My fix is an odd one: take out the old encoder of the x15 (disassembly guides are online), buy a similar enough mouse encoder from ebay or aliexpress, swap the new encoder with the old encoder wires with soldering, and reassemble. EVGA voids the warranty doing this however.

seishou
The cause seems to be from a bad scroll wheel encoder; unfortunately I asked EVGA support about what model of encoder was used and they do not know.  They claim it's a standard encoder, but only 1 result on google will show a similar looking encoder as the one used in the x15.  
 
My fix is an odd one: take out the old encoder of the x15 (disassembly guides are online), buy a similar enough mouse encoder from ebay or aliexpress, swap the new encoder with the old encoder wires with soldering, and reassemble. EVGA voids the warranty doing this however.

I was wondering about this. I wonder if the g600 will work. I think I still have that around I could take it out of.

As said the fix doesn't stick after a bit and I tried a new one and it has the same issue. I think it does need a whole new part. Everything else about the mouse works fine. Dealing with RMA sucks and they'll put the same type parts back in, they don't do upgrades. Like I had a laptop where the HDD went bad 3 times under warranty because they just replace it with the same cheap part as original.(different company but they all work like that)

Well my RMA Z15 work fine for couple months now this one is having issues scrolling too. ate least had like 2 months of happy mousing.

Hi guys! I can't create a new topic, I wrote a repair manual! Suitable for those whose mouse is not under warranty. In Russian, but you can use Google translator.
 
docs[DOT]google[DOT]com/document/d/12aOMJw7NAXwXU55Wnyp5VFz5tpXm7JjPdL59xc5CQ2A

iG0Rs
@stomiroslav
Thank you. It works great.
Дякую.

You're welcome! I'm glad it helped someone!
Пожалуйста! Береги себя

Just want to say that the soldering joint to the mouse wheel is TERRIBLE.
 
This is bad QA.
 
I could RMA my mouse with my local vendor but the travel time is way too long to make it worth the trip to the vendor's office, so I open up my X15 and saw that one of the wires connecting to the scroll wheel has already broken off. The soldering joint looks non existent. 
 
A quick solder and it's working now.

Hoggle
Glad you got it fixed since it’s a good mouse I feel it’s a great mouse for the price. So far mine have been good. Of sometimes solder can be bad for some products produced on one day and not other days. It could be that for some the solder was running low and didn’t get replaced soon enough.

Regardless, that would mean bad QA. I expect the mouse to work longer than the one month I had it.

Owner of the mentioned YouTube video. I created the video to demonstrate exactly this, because while EVGA accepted my RMA (times 12 or 20 by now), they claimed ignorance on the issue, at first, and said they couldn't reproduce it in-house. I started with an X3 from Best-Buy, RMA'd that something like 8 times, got a "free upgrade" to an X5, along with a "free" mouse pad, RMA'd the X5 maybe another half-dozen or so times. I have three of these mice, at this point, though, because when the scroll wheel DOES work, it's an absolutely AMAZING mouse!! Easily the best design in the industry. I just wish assembly matched the design. Of the three that I currently have, one has the scrolling issue, one does not, and one is physically worn out and broken (the right-click cover is cracked almost all the way across, and ready to fall completely off the mouse, as well as the switch underneath it and mouse-wheel-click being nearly unresponsive).
 
One solution that I found was to carry the mouse back and forth to work with me. Plugged into my work computer, it's like the mouse would "learn" over the course of an 8 hour day how to behave. The jumping would start out the same at the beginning of the day, but over 8 hours, it'd slowly go away. I'd take it home at night, and it'd slowly come back. Kinda imposed a semi-reasonable time limit on my evening gaming sessions: I had to stop playing when the scrolling became completely unusable. Turns out, work is touchy about things touching their computers, so that nixed that "solution".
 
Another crazy "solution" was when my GTX 980M burned out, and I replaced it with a GTX 765M (because this was already the third graphics card in 4 years and I was tired of spending $400 at a whack), the new graphics card made the problem go away instantaneously. Problem when GTX 980M shut down; zero problem when GTX 765M started up. Using that computer hasn't fixed any additional mice. Just the one that went through the transition with it (which is now the broken, busted, and completely worn out one). It's almost a shame the warranty period doesn't reset when you get your RMA, because that broken, busted mouse didn't last 3 years from when it showed up on my doorstep, but it was received with days left on the original 3 year warranty from the very first X3. I think spending every day in my pocket was hard on it.
 
For the record, I've seen other threads on EVGA's website dated 2015 and 2016 citing this same issue, and my first RMA was in the 2016 timeframe, so this isn't anything new, and it's not something EVGA is losing enough money on in RMA's to bother actually fixing. Which is weird to think, when I'm here doing 5 Cross-ship RMA's back-to-back, and even RMA the NEW mouse, because it's jumping worse than the one I'm trying to replace (that happened a few times).

EDIT: looking at my profile info to the left of this post, I see "joined: 8/2016". That was specifically so I could RMA that first X3 mouse from Best Buy, when it developed this issue after maybe 18 or 20 months.