I'm having an issue with this as well.
I have a 1.4 Athlon that's underclocked to 1313mhz because it tends to hover around 70-77 C at full speed. I've tried a few different fans, and I'm still having problems. Do I maybe have bad heat sink paste? I'm only using a very thin coat.
BTW, underclocked with no load it runs about 59-65 C. Things seem to run ok, but I noticed a slight shaving off the top of the chip itself a few months ago. In fact, the only lockups I generally get are video related (IE, I switch from Jabos to Glide64, and I ususally wind up having to reboot).