The sheer incompetence of some "professional" repair guys is sometimes mind boggling. Usually you end up replacing parts that weren't the problem anyways. Ofcourse you shouldn't use 450 watts where it says 500 ...
What processor are you running with that video card? And a tad of a personal question: How clean is that pc? Fans clogged up? From my own experience i found that at least 6 out of ten times, heat is the culprit.