What kind of power supply do you have? Possibly with your CPU overclocked and the card requiring quite a bit of power your 12 volt line is straining? Whats your wattage and Amperage per rail meaning +3.3, +5, and +12 volt.
BTW what nanowarrior suggested isnt too far off from the truth. Could be a heating issue especially since the thing is overclocked. Download the Coolbits registry file and it'll allow you to check your clock frequencies and maybe bring them down a little.