I believe this prolbem is actualy difrent.. from my research.
Ive got two ideas. first off is, nVidia is trying to auto set your display and searchs for the proper mod lines. I will try the Option "UseEdidFreqs" option to force it to use mine...(we force it, because after all its MY monitor, I can test it here much easier than nVidia can test my monitor.. right? also they may have a "protection" method in there to keep me from breaking my monitor when it detects an illegal mode??) so anyhow.. tuneing your modlines "might" help, but I really dont know what the card would like... if only we could tell what it decided to use, and plug that into the XFree86-4
and the second Idea I have for crashing/slowness is the incompatabily prolbems with AMD, Athlon's and Linux's (and nVidia's as well) AGP controler.. the prolbem here is in the way Linux handles memory.. its not really a prolbem with either AMD nor nVidia, and the Linux kernel team is currentaly working on it. the "fixes" may be in the development branch so we may not actualy see a stable AMD AGP driver till the 2.6 kernel. a similar prolbem existed on Windows 2k..