unable to overclock the evga gtx 260! help greatly appreciated

I recently bought a gtx 260 evga superclocked edition, It seems that most people are getting really high clocks on this card but if i try to overclock using multiple oc'n software, once i launch a game or benchmark the game crashes and shortly after the computer reboots, I'm using a BFG tech 650 watt and im not currently overclocking, so far i have tried going to 650 core 1200 mem and this will not work, anyone have any idea as to why this may be happening?

Have you tried this. It works great for me. http://www.evga.com/precision/

Yeah the EVGA precision tool is pretty much fool proof. Keep in mind some cards will overclock to the sky and some won't overclock at all. It's a total crapshoot.

or if u want to go a little more advanced, go with RivaTuner