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#11
Ahh, the disappearing RAM trick. The problem is definitely hardware related, not software. It's not uncommon with the X58 chipset, especially upon a cold boot.
When you ran memtest, was that only on with 4GB detected by BIOS?
Some Common Known Causes:
Faulty RAM module
Incorrect RAM timings
Faulty Dimm slot / mobo
Faulty PSU
Improper CPU seating / uneven pin+pad contact
???
I'd start with the simplest route, which is eliminating a faulty RAM module or dimm slot.
Remove all but one known good 2GB stick, and then boot the PC from each white slot individually (or dimm 1/3/5). If the PC POSTs from each slot with a known good module, try the other 2 modules individually.
If the PC fails to POST with a particular RAM module in any dimm, then the RAM is faulty.
If all the modules are okay but the PC fails to POST in one dimm slot consistently, then you have a faulty dimm slot.
If there is no problems with either the RAM or the dimm slots, then the issue lies elsewhere.
I've had similar issues with that with my current build. The first time it occurred, one of my memory modules had actually died. But then it also happened with my replacement RAM, yet all the RAM is good and so are the dimm slots.
In my case, it wasn't until I removed one of my suspected to be faulty video cards that everything has returned to 'normal' (ie I consistently show all 6GB at POST now. Before, it would sometimes show all 6GB, sometimes only 4GB etc. However, I still can't conclusively say it was the video card causing my issue )