PLEASE HELP :'(

well here are my system specs:
Intel Core 2 duo E8500
Geforce GTX 260 overclocked to 700mhz runs COOOL at 65 degrees celcius
4gb PC2-6400 800mhz ram
550W coolermaster realpower PSU
Gigabyte P35 Ds3l something motherboard

pLEASE HELP

heres my issue
i constantly get the Error: Display Driver has stopped working and has recovered
Im playing a game such as GTA IV or Crysis oR ANYTHING and as im playing (FREQUENCY IS VERY RANDOM) the game freezes and it keeps playing 1/4 of a sound it was playing right at the moment before the freezing (like tatatatatatatataatatata), screen goes black and when i control alt delete out of it i see that message
sometimes the monitor stops detecting my computer and RIGHT after i get the blue screen of death

ive searched everywhere online for the solution...i turuned off UAC and problem got better but now its mUCH WORST
my system used to be overclocked to 3.8ghz and i thought that was the problem so i went back to the default settings and now the problem is worst than ever

my CPu runs at 60 degrees celcius when playing Crysis and my Gpu at like 65

i have the latest Nvidia Drivers (big bang II?) 180.48
please help me
PLEASE IVE SPENT OVER 1200 DOLLARS INTO THIS SYSTEM AND I CANT DO ANYTHING WITH IT BECAUSE OF THIS PROBLEM

i only have my Gfx card OCed right now and i did it through EVGA precision

i think i fixed it
i made it back to the original factory overclock at 620mhz and i dont think it was getting enough voltage at 700mhz? lol
the memoryclock was also faster though...