Couple of suggestions
- Try running the RAM at a lower speed. My DDR 400 seems to play silly buggers if I run it that fast, so I run it at DDR 333. Regardless, there's no real point running above your CPU FSB if you are usin Dual Channel on an XP. There's not enough CPU bandwidth avaliable.
- Try opening the side of the case and pointing a desk fan at it. If this solves yur problem, get a couple of fans for your PC: a 120mm rear fan and a 90mm front fan should do it.
- Clean off the Thermal paste and apply only enouch to make contacy accross the core. I had no end of problems when I worked at Studentcomp with overly pasted CPUs.