AMD Phenom II X2 555
G.SKILL 4GB DDR3 1600 (Model F3-12800CL9D-4GBNQ)
Sapphire HD 6870
Western Digital Caviar Black 500GB
Alright so I'm having a very strange problem and honestly I'm completely lost about what the cause is in this case. I built this PC just over a year ago and somewhere around the past three months this problem has surfaced.
If I leave the PC off overnight (any any extended amount of time) then boot it up the next day it will post and boot into windows just fine but once there the problems start. Following the first boot after being off for awhile memtest will show countless amounts of RAM errors BUT if I switch off the computer and change the RAM sticks around then boot to post screen and repeat this process about 6 times (lol) the problem will eventually fix itself and I can run memtest for hours and receive no errors Is it the board or the RAM
If you have tried the same RAM in another PC and run Memtest with no issues, I'd say it's the board. If you use different RAM in the board and have no issues, then it's the RAM. If it like messing with you and doesn't want to play fair, it could be various issues, like RAM incompatibility or even a standoff in your case being in the wrong place, shorting your board.
An issue I had a while ago was that my RAM didn't work in my board, but worked in another board, but other RAM worked in my first board just fine. I eventually breadboarded the thing and it all worked fine, but as soon as it went into my case, it ceased working. I eventually found the main issue: a case standoff. It was slightly too high, and when I had all 4 sticks in, it caused a short. The second issue I found was that I had to force my RAM to run at 5-5-5-15 (default for this RAM) or it wouldn't work.
Try breadboarding, if it doesn't work, scream at it and try again.