I got a similar Problem, but a bit different. In single Monitor use, all was working fine. In TwinView Mode, X crashed after a few Seconds. Most of the Time i can move the Mouse, but cant do anything else (CTRL+ALT+F1). So i have to hard reset the Computer.
I changed my XF86Config-4 in Section [Device] :
Option "NvAGP" "0"
After this change, everything works fine.
So im sure, it was a problem with the AGP.
So please put also your Bios Informations for AGP
like "Allocate IRQ to PCI-VGA" in this Thread.
Its not Mandrakes Fault if its not running on your System.
I use the NVIDIA Driver since Mandrake 9.1, and it always worked.
p.s.: Im using Mandrake 9.2