About the PSU- I'm a little confused on why it would stop working during high-video demand but not any other time. Isn't the power draw the same either way?
GTS 250 have 2 different power draws. On the idle, it draw about 150W and on full load, it draw about 250W.
What are the details of ur PSU ?
If ur CPU temp is ok, then, i suspect it's ur PSU.