Have a look at dmesg, is there anything else using the same IRQ as the graphics card?
I had a similar issue with the onboard ethernet in combination with an FX5200, had to use a pci ethernet card so I could assign it a different IRQ. The nvidia driver should
be able to use a shared IRQ but I've never gotten it to work.
If the machine hangs are you able to log in remotely using ssh? That way you can at least reboot the machine properly