You can't be sure, but should try to make sure it's nothing else first.

Doublecheck all jumper settings first..
Then check the following, as they can all affect stabillity:
CPU not overheating.
Northbridge not overheating.
CPU core voltage.
Memory voltage.
AGP card not overheating.
AGP card voltage.
Memory timings (Start with the least aggressive automatic timings in the BIOS, not optimised)

All of these measurements should be readily available from MBM (someone linked this earlier).
Something else you can try is Sisoft's Sandria utility. The freeware version is limited, but still useful in your case. You can run a burn-in test, and select which part of the system you are stressing, ie memory, CPU etc.

The amdmb foruma should be able to tell you whether people have gained stability by eg upping memory voltage. I know that on my Epox 8k7a, upping the memory voltage was necessary to gain stability when using optimised settings.