You say "3x successful memtest86 on all ram" is that 3 passes? You should let it run overnight, or for at least a few hours to be 100% sure.
What are the temps in your system?
will give you a general idea what your temps are.
If anything exceeds 60c when sitting on the desktop that could be the problem.
Silly question, is everything seated properly? I have been fixing computers for people for more than 10 years, and I had this problem just yesterday. My graphics card wasn't seated properly and it was rebooting/crashing.
I don't know what the problem is, but this will at least narrow it down a little.