you also may want to try a friend's psu if they have one powerful enough for your system, you really want to try to eliminate as much as you can from the equation...
ive tried 3 or 4 diff power supplies some stronger some weaker, still nothing. ive also went with only one stick of RAM switched between all 4 slots, the CPU fan is running and at a slow speed, the video card is also running at a slow speed but it still has the jumps in it like i described close to the beginning of the thread...