Thanks for the ideas.
Tried all the different NvAGP options, but to no avail :-(
I also went back to 4496 because I read in this forum that people were having more luck with that. And I set the went back and loaded the fail-safe BIOS settings.
Does anybody have a Shuttle SN41G2 and got the GeForce card working stable ? I am willing to use a different linux distribution if necessary...