I have a similar PC with you, and I'm using ASUS 6600GT also (256MB PCI-E). So far I have no problem like this in MTA, but usually this is caused by overheating of your graphic card. Try to open your PC casing while you run it. If it still appears, maybe its some other issues.
Thanks
Note : Actually your system is not very fast for san andreas, mine is Intel P4 3.2GHz, 1GB RAM (Kingston, but forgot which 'type'). When I play GTA:SA with full graphic settings, it also lags sometimes. Well, now I am waiting for 8600 Ultra (the price seems to be around 200USD). Its a lot better than my 6600GT, and most importantly, its DX10 compliant card