I had very similar problem and I have very similar system. I have E8500 and GTX280. Thing that fixed the problem for me was when I matched FSB and memory frequency which meant that I increased FSB (QDR) to 1600Mhz and that meant that FSB was increased to 400Mhz. I also decreased multiplier to match the old cpu clock as closely as possible. In your case it would mean 7.5 multiplier. When FSB and memory is matched it generally means more stable system.