i am tired of buying the wrong parts.
I totally know how that goes. I just got done repairing a system on which the CPU was dead, but there were really no symptoms. Up until a couple of days ago, I would have told you that it is likely only three things; motherboard, memory, and video card. After the experiences of my last few weeks, it turns out a defective cpu can cause the problem also... <sigh>.
Anyway, about the only way you are going to diagnose the problem is by swapping out parts. Does the board have built in video? If it does, hook it in to that and see what happens. Yes, I realize that you probably won't be able to play any cool games, but the idea is to isolate the video card as the source of the problem.
When the monitor kicks off, are all of the fans still running?
Gosh, I don't even know where to start with this one...