Which games it this happening in?
Yeah we need to know what game this is.
ALSO, have you overclocked this card at all? Do you have sufficient case airflow? What kind of cooler is on the video card? is the fan on the card running? I had an issue way back when i had a 5600FX and the fan died and it would lock up after a while. How far into the game is it that it locks up?