Just had a look at the specs for your PSU now and it has two +12V rails that supply 14A & 15A respectively. Your 8800GT requires 9-10A to run it, but you say you've overclocked your graphics card so the actual figure is probably a bit more. I'm assuming you have overclocked your CPU too since you have an aftermarket cooler. Perhaps there just isn't enough amps on your +12V rails of your PSU to run your system adequately. I'm no expert on PSUs so maybe wait and see if anyone else can offer some further insight.
Although I would definitely try with onboard graphics and see if its any better. Maybe also try your GPU at stock speeds.