If this recently happened it could either be an update, cache or hardware issue (which I highly doubt).

-An update
If you recently had a windows update, updated gfx card or third party driver update you will need to check for any conflicts in event viewer or device manager. If no problems then proceed to cache.

-Cache
This one is tricky because I am unsure of where all of css cache goes to, but the main area that I know of is...

C:\Program Files\Steam\steamapps\Your STEAM ID\counter-strike source\cstrike\cache

rename cache to cache-old and re-launch steam and play css normally, it should create a new cache folder. If not delete the cache folder (Not Perminently though just incase if any conflicts occur with your steam client).

If this still fails you might want to try a full re-install of css. Just delete local game content when you click on properties of css in the game list. After deleting local game content delete your counter-strike directory.

C:\Program Files\Steam\steamapps\Your Steam ID\counter-strike source folder.

Re-launch steam and download the game content again, just to be re-assured just clean out your whole computer while css re-downloads. Run CC Cleaner or your prefered cleaning software. Once your pc is clean and you downloaded CSS and start the game again and re-configure your whole game, this should be fixed. If not, you will then need to look at the hardware level or server level (Which I highly doubt). There are possiblities where ATI products malfunction on the game which is modified (Warcraft MOD). Valve did impliment auto-hardware detection which dynamically configures your display settings depending on what graphic card is found by counter-strike engine. If the game is modified out of the range of which is normal there could be a graphic conflict which is pretty hard to fix unless you modify your display settings manually. Try to change your graphic settings less and high and see if this fixes your issue. I do apoligize if this does not resolve your issue.

I hope this helps,
KOOBERZ