Mem
Lifer
I would say ram first then CPU,btw I had a faulty cpu(T-bird 900) like that,infact it was rock stable for 6 weeks then started playing up. I went through the ram first(sent it to crucial for testing and it passed) then finally tracked the problem down to the CPU,yes rare but it can happen . Normally motherboards have more chance of going fautly then CPUs.