Re: System freezes completely - RESET-Button only way out
Please generate and post an nvidia-bug-report.log for review.
If disabling AGP with NvAGP=0 seems to eliminate the problem, then you could be hitting an AGP driver bug, rather than an nvidia X driver bug. You could run spec.org's specviewperf for an OpenGL benchmark. You could also try reducing the AGP rate from 8x to something lower and see if that has any impact.