FPS drops fiercely when playing

Good day!

I am using EVGA GTX660 and always face FPS drops when playing for 2-3hrs, under the highest graphic setting.
There is a message said the system can not find the VGA driver when it happens, but it become normal when I restart the game. I don't face the same problem when playing other games, even in the highest graphic setting.

Now I tried to lower the graphic settings, however, the problem still happens.

This is very annoying, especially in the dungeon.

My hardware is as following:
OS:Win7 64bit
CPU:I5-3470
DRAM: 8GB
VGA: EVGA GTX660
HDD: Plextor M5S 128G / WD500G

Any suggestion to fix the problem?

Thanks in advance :)

dl advance system care and the game booster that comes with it
both programs have a button to switch off all backround programs..


im guessing u have ur grafix card set to all the right gameing settings
gfx cards dont come rdy for gameing if u didnt know. u need to set them up to push the power