Sudden Lock-ups?? Help!!!

[fordracr]

OK I have been running my comp fine at an overclocked speed of 700@854 for like a couple months. However now sometimes when I play quake my computer suddenly locks up, and my screen goes black. WHAT THE HELL?? I put the speed back to 700MHZ has anyone any clue?? I think it could be a power supply issue. However I got an Antec 300 WATT which are usually very reliable. Please give me some insight. THANX

[Pinky]

If it doesn't lock at default cpu speeds, then it was the overclock... make sure your cooling is still working and that warmer room temps aren't ausing your case to be hotter than it was.