mem stuff
Do you have options to change the ram timings...cas latency, bank interleave, normal, fast, turbo, etc??
Try to get the latest bios version for your board also... a lot of times there are performance bioses or new features are added in newer bios versions.
Without mem voltage adjustments, you can still burn-in with changes in the timings and FSB, but voltage will be a big help to get the memory to run higher FSB's and to make the burn-in more effective.
There's no telling 'till you try it out... weaken upthe FSband timings a bit if possible, and loop some Sandra memory tests for a few hours... gradually increase the settings/FSb and keep at it. Maybe it'll help. Consider looking up a voltage mod for your board's memory as well.