91.31 drivers. GPU fan runs (when over 50C). GPU is typically around 50-54C normally (including when decoding video streams). Not sure of the temp in-game when the freezes occur, but after exiting the game typically it's not high (forget the actual number) - Rome Total War doesn't exactly stress most 3D hardware. And I can get freezes almost immediately, or 15 minutes or an hour into a game. Typically in-game freezes resolve themselves within 1-5 minutes, and often audio plays for a while (or gets stuck in repeat). Video-decode freezes typically never recover, and usually audio stops dead).
I see in other posts that 91.31 has caused other people lockups while watching video off the web.
You might try using one dimm in slot one and run your system that way and see if the lock up occurs. You might consider using an older driver version than 91.31. RAM is often the cause of system lock ups.