Re: GL jerky then freezes
Have you tried both the nvagp and the agpgart? If you haven't switched, I would try the one you currently aren't using.
If you are using:
options nvidia NVreg_EnableAGPSBA=1 NVreg_EnableAGPFW=1
Disable each of these options and see if that makes a difference.
If you hadn't mentioned that you had this issue with different cards, I would say overheating. I had this problem before, and it turned out my GPU fan was kaput.