Before you go for a Radeon XFree86 4.2 still doesn't support very well.
We have had some problems at my office with a GeForce2 on a KT133 motherboard (not exactly what you have but ...) . Two things you might verify:
- in the BIOS, try 0xDA for the AGP driving value;
- ensure your refresh rate isn't below 75Hz, with value lower than that we observed periodic freezes of the machine (this threshold value might well change from machine to machine).