I recently got my hands on a used X58 Classified3 the E770-A1 Model, with 24 Gb of Corsair Ram and the i7-990X.  I was told that the system was having some issues. Seeing that I have the orig X58 SLI running a 920 with 24Gig of Mushkin ram, I figured the system would make for a sweet secondary.
 
Now on to my problem. I am not sure if this is a motherboard or cpu issue.  It would seem that when I boot into windows both windows and CPU-Z will see all 24Gig of ram. Sadly in windows it says that I have 24Gig (16 Usable). I did some looking and checked my low memory gap and even pulled and reseated the ram. None of this worked. So I figured sense I have Known working ram in my system I would pull and use it. I started with all 6 sticks and got the same issue. I then pulled 3 and would still show some of the ram usable. When all was said and done I got down to a single stick in DIMM 1 and the system would not reboot. It went into a reboot cycle that required me to power the unit off. Further testing showed that DIMM slot one was the reason for some ram being unusable and then when further testing showed that DIMM Slot 2 would add to the amount of RAM showing unusable.  These DIMM are side by side, I do no wish to go threw the trouble of pulling my fully working 920 and the 990x to  swap them out to verify that it is not a cpu issue. Granted if this is advised I will do it to know the exact location of the problem, I was just hoping that maybe someone here could tell me if its the CPU or MB without me going threw the trouble of pulling apart 2 systems any further.
 
Thanx for the help everyone in advance

Currently I'm running Windows 10 64bit, clean install.

Yes I followed the rules. As stated when I populate DIMM Slot 1 the system just continues to reboot as if it is unable to fully post.

When I fill 1 & 3 it posts and Windows shows 8Gb (3.99 usable)
When I fill 1, 3, & 5 Windows shows 12Gb (8Gb usable)
When I fill 2, 1, 4, & 3 Windows shows 16Gb (8 usable)
After that the amount continues as if the system is holding onto 8Gb of ram.

This is why I assume there is an issue with slots 1 & 2.

I have six 4Gb sticks total 24Gb of corsair that came with the system. Plus I have used the six 4GB Mushkin ram I have in my X58 that will show all 24Gb in Windows

Thanx for the information, I wonder if it's even cost effective to try and have the board fixed.

I guess I need to talk to EVGA about costs and the such, or I guess I can just keep using it like it is until it fully dies..