If you are using WinXP it may be that the refresh rate on your monitor is going really low.
If it is the monitor that is doing this, try this:
http://downloads.guru3d.com/download.php?det=354 Refreshlock is the program. Make sure it is set-up properly.
If it is not the monitor, have you tried the latest ATi drivers, i you have not try the link in my sig. (Catalyst 4.3)
I would try the Graphics drivers first (I would do this anyway if you don't have the latest ones), then if that does not sort the problem try the RefreshLock (assuming it is your monitor)