A few questions then ill tell you my story .Did this sytem come with the FX card ? Have you changed anything recently and then started having problems ?
Sense you havent given much info on your system i can only suggest from experience.The problem may be a setting in your bios.Boot to bios look in Advanced /Chip Configuration.
In the line Video Memory Cache Mode see if your Configuration Option is UC or USWC .
USWC(uncacheable , speculative write combining )
is a new technology for the memory video of the processor.It can greatly improve the display speed by caching the display data.You must set this to UC (uncacheable) if your display card does not support this feature .
My problem at the time was while running a 9700 Pro card my performance was CRAP .3Dmark01 se yeilded a terrible score around 9000 .Not to mention the fact that games just plain SUCKED .Needless to say i was mad as a hornet on steroids.
Then i relaxed and went through my stinking manual troublshooting the system and found the problem was the
Video Memory Cache Mode .it was set on USWC so i selected UC and solved the problem . my 3DMark score yielded a 16500 .
Running the computer in any other manner would not reveal this problem it worked fine .But when game time came along it was terrible . Hopefully this will help you chek it out at least