A reboot usually indicates overheating, a lack of power or a configuration problem:
-Overheating: Open your cpu cae and have a fan blowing inside while you play. If it fixes your porblem, it was overheating.
-Power supply: Open your case and look for values on your power supply. You need at least 20 amps on your +12v rail if your 3d card uses external powering (like an electric plug that fit on your card, inside your comp)
-Config: If its an AGP card:
-Change AGP from AUTO or 8X to 4X
-Disable AGP Fast-write