What's in the /proc/fb file? If the file doesn't exist, then passing vga=788 (for a 1024x768 framebuffer console) has helped a few people with sync errors when exiting X.
If the file does exist, then try passing vga=none to the kernel, and see if that helps (though I have a feeling it won't).
And the video BIOS version would be a good thing to post, since Andy asked for it. I'm sure it'll help nVidia isolate the problem.