Okay Pape, doing some Googling, I see that others with that model video card have had driver-based gaming problems. Here's a solution that was posted at HardOCP that worked for some users:

Go here, download this:

http://www.guru3d.com/category/driversweeper/

Firstly, go to your device manager, find the display adapter listing for your card, click on it, go to the "driver" tab, and use the bottom radio button to uninstall your drivers there. It WILL bork up your display (icons may double, you'll be at godawful resolution, etc), but it's natural for that to happen.

When your drivers are uninstalled, BEFORE you reboot, also run that program. Run it twice to be sure. Make sure that you select "ATI drivers" in the menu, then click "Analyze," then "Clean/Sweep/Whatevercommandisheretoexecute."

Then reboot, cold boot preferably, ie total shutdown, wait 5 seconds, power on (clears laggy ram and capacitors). Boot up, install your drivers, reboot.

This has solved 80% of driver issues with ATI cards (and some nVidia cards) that I know of. The other 20% mostly deals with registry values and, in absolute worst case scenarios, hardware issues or a complete Windows reinstall.