That sounds like a heat problem.
To verify, Try turning the graphics settings way down
- or -
open the case, point a stadard type hopuse fan directly into the guts
and run the game. If it runs fine like that, it's more than likely your CPU protection kicking in and shutting the machine down when it hits the threshold temp.
You might also want to grab a copy of
SpeedFan and monitor your temps directly.
JD