Eagl,
Pardon the obvious, but is your AGP running at 66mhz? Your motherboard should have settings for full and 2/3 speed for the AGP.
I am running a 5500 with a PIII 650 @866. This yields 133mhz and when reduced by two thirds, I still have an AGP speed of 88.6. Things run pretty smoothly most of the time. However, once in a while when I really load up the graphics (1280x1024, 32 bit), attacking a buf at close range with guns blazing, my display will freeze for about 3 seconds and then return to normal (if I am still alive). I'm working on troubleshooting this problem, and one of the steps will be to bump my CPU speed down a bit so that my AGP is closer to 66 mhz. You might try this test just to see if it helps.