You were clear about WoW load, I made a wrong assumption. I hear what you're saying about temps and tests. Still we're left with the question, "What changes in a computer over a three hour session?" Unless you can think of something else, we're left with the answer, "Temperatures and memory allocations." Heat has more than one effect, and there are two types of memory in the machine (video and system). I've read that heat can make cards shift in their slots. Did you try pulling the video card out and shoving it back in?

If it were me, I'd do the reboot test I mentioned. If a reboot makes the machine good for another three hours, then it's not heat. I would trust that test more than thermometers.