I was having this problem also -- got the game to where I could play for a while then had to 3-finger-salute and restart when it stopped responding.
Athlon 64 X2 3800+ - Nvidia 6600 w/256MB
The nvidia graphics fix mentioned at top reduced problems for a while but they still happened...
Then I did another web search because it was becoming uplayable again..
Found THIS solution and now problem has gone away.
1. Check your "My Computer -> Properties -> Hardware -> Device manager"
2. One of the items listed should be "Processors"
3. Double clickon either one of the processors - go to the driver tab - look in the "Driver Provider" area
4. In my case the provider WAS Microsoft...can't recall the driver version just now.
5. Went to this page at AMD site and downloaded the AMD dual core CPU driver.
6. Once installed (steps 1 through 3) show a "Driver Provider: Advanced Micro Devices" and a "Driver Version: 1.2.2.0"
7. Have played now for a whole planets worth of quests and no lockups so far.
8. PS: I am thinking of undoing the nvidia OpenGL fix above and see if the graphics effects come back...if not then it might even be faster (allowing more simultaneous gfx threads)