it appears to be most 5200 cards and some of the other fx based cardsare effected. I went from a geforce2 200mx to a 5300 ultra on the same machine/mandrake install and ran into all of the same problems you have had. I have gone through all of the reformatting, kernel changes, adendums to grub, with no change in results. What I didn't try was down gradeing to a 2.4.20 or below kernel. (seems to be a 2.4.21 problem).
Hopefully a new driver will fix this.
I'll be testing 9.2 beta later, after i recompile the kernel to match the gcc version so i can get the nvidia driver to work.