id check the power supply first, just for elimination, got my wifes 8800gtx doing the same thing under load, new psu with dedicated rails sorted that issue out.
He's already tried it with a different PSU...
Please remember that any mention of competitors, hinting at competitors or offering to provide details of competitors will result in an account suspension. The full rules can be found under the 'Terms and Rules' link in the bottom right corner of your screen. Just don't mention competitors in any way, shape or form and you'll be OK.