EVGA_James
Unfortunately I have not yet been able to recreate this with a several of my X15. Our team is looking into these issues and should come out with a update soon. If you are still having issues please contact our support team for a warranty replacement.  

elvinmagical
I was able to downgrade my mouse firmware with the following steps:

1. Download EVGA Unleash RGB software version 1.0.13.0:
2. Extract the EXE file to a new folder using 7-Zip
3. Open Library\X15RGBLibrary.dll with a binary-safe text editor (Notepad++)
4. Search for the string "X15_v1.28.bin", which is the last good firmware file as of now (the file itself can be found in Firmware folder)
5. Look for the number after file name ("284"), it's the firmware's internal version and compared with the one from connected mouse
6. Modify it to become 291 (the latest/buggy firmware v1.34 has internal version of 290, according to Library\X15RGBLibrary.dll found in driver version 1.0.15.0)
7. Quit any running EVGA driver, run extracted UnleashRGB.exe, it will attempt a firmware "update" (downgrade to v1.28) because mouse-reported firmware version is 290, lower than 291
8. After finishing the update, the driver will attempt to update again (this time it's 284 < 291), cancel it
9. Voila! Your X15 mouse firmware is now downgraded to v1.28.

The 1.0.13.0 driver can then be installed normally. Don't forget to disable auto-update this time.
(EVGA, please don't patch it in a future release. We need this trick to prevent other undesired firmware updates done accidentally.)

drew2541
running firmware v1.42 for x15 for 24 hrs haven't had an issue now

zapoqx

rahorton
I am also having the same issue. I wanted to go the RMA route. However, the X15 was my only mouse so I opted for the advanced replacement. They wanted $166 hold on my card which seems insane. I can buy the mouse new for 40 bucks on Amazon so I declined against it. I will try the update posted here that is supposed to fix the disconnect issue. 
 
I have typically had no issues with any EVGA product but I have to say the X15 so far is not impressive. I ended up getting a cheap $20 RGB mouse from Gamestop that has been far more reliable. I will see how the update posted here works and report back. Really dissappointed on this one hopefully the update will turn it around.

I get it.  The X15 sold me when I had comparisons to the Logitech G600.  I didn't have disconnect issues, but I also didn't update to that version that had the issues people were experiencing.  Some claims for disconnection included AMD systems with that bug in the BIOS, but again, I never experienced it, probably because at the time, I didn't have PCIe GEN 4 stuff to initiate the issue (a common denominator).  Now that I do twice over, I have yet to experience anything of the sort.  Having the X15 for a short time and having an issue with it was surprising and semi disappointing, but, like the Z20 Keyboard, there was a few small software issues I had that got corrected with firmware and software updates and I have yet to be disappointed since so the X15 is just a matter of patience in the same way.

Also, $166? Sounds insane!  I went with regular RMA just for the scrolling issue, though I was surprised I got it back at a reasonable time, though I had my G600 to fall back on (even if the scroll button was horrible).  Getting back the X15 has been wonderful with everything working!  Hopefully, your experience becomes better.