System crashing, graphics card/driver problem

Xttony

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So I was trying to overclock my processor and the overclocking session freaked my GPU. The system crashes occur randomly sometimes when idling sometimes when playing video and sometimes when playing games. But sometimes the system is very stable and does not crash at all even when playing games at max detail.
After some search I've narrowed the problem to the graphics card drivers/other drivers or the card itself.

So can you guys point me in the right direction, and tell me what to fix.


Note
I was overclocking my old processor which was coupled with my old GPU but now I've put the new ones back in and the problem persists.
Checked temperatures and they are normal.
CPU, FPU, Memory, etc stress test successfully passed but crashes when stressing the GPU out. But at times runs all heavy games easily.

Edit OS windows 7 ultimate SP1
 

Xttony

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Forgot to mention I overclocked my old processor after I bought a cooler. But now using my new processor at the stock clocks and stock voltages. When I overclocked my old processor I increased the voltages too.
PS. There didn't seem anything wrong with the overclock and it passes the stress test(CPU and FPU) for more than 3hours.