Because your problem is so intermittant, its impossible to tell weather or not the CPU is ok based on those tests. At this point, I'd have to say the problem is one of two things, the CPU or as you mentioned, the PSU... Unfortunatly though taking either one of those pieces to someplace to get tested isn't practical, becuase like you said, it at times works fine for a couple weeks. You'll probably need either an extra CPU or PSU to be able to adequately test it.