Problem with Modern Warfare 2

[Loud_Silence]

I've completed Modern Warfare 2 without any problems and now i'm playing some levels again, but i have a problem. During the level "Wolverines!", sometimes the screen starts flickering. Sometimes the game freezes for a moment, sometimes the flickering stops and nothing happens, and once the game crashed with a message saying something like "3D device not accessible".
It happens mostly when using thermal sights or Predator Drones, but now it happened me twice while on "normal vision".

I have a GTS250, drivers updated, and my system meets the recommender requirements for MW2. Any ideas?
Thank you

UPDATE: i've been testing the thermal sight and the Predators on other levels, and so far i haven't encountered the problem again. Maybe it's related only to Wolverines! (it's a pretty large scenario).

The graphics card (nor any other component) isn't overheating. The GPU never goes higher than 70ºC, and the CPU is always below 55ºC.