first i would try raising the Vcore, if that doesn't do it then try raising the Vdimm.
also, check to see what CAS timing your ram is running. if it's at cas2 then set it to 2.5 or 3 before raising the voltage. if the system is stable after you raise the cas timing then the ram was the culprit.
oh yeah, what FSB are you running at when it becomes unstable?
the PCI bus usually starts to crap out around 39Mhz unless you have good overclocking cards (the PCI bus will usually give you problems before the AGP will)