I realize this was six months ago, but did you ever try replacing the heat sink grease between the CPU and the heat sink/shield? You mentioned looking at it in a mid-thread post, but never said (I think) that you tried anything with it.

Your symptoms could be a number of things, but they're consistent with overheating, and if there's no heat sink compound actually on the CPU die (raised rectangle in center) then there's a good chance that it is overheating.