i've got similar issue, have posted here before about it. still exists though.
P4 3.0 800fsb prescott chip
asus p4p800se mobo
ThermalTake fan, manual control, never less than 5000rpm, w/AS5
core voltage at default 1.38-1.41v
had 2x256MB RAM, but one burnt (or whatever) so i'm only running 256 total, default voltage also 2.75v
AGP voltage at default also 1.8v
idling right now between 59/63C. running media player w/audio/video brings it up to 70C, same for games. occasionally spikes up to 90+C for no reason. doesn't shut down though! btw i did an immediate shut-down. i surf the web a lot, and play a couple games (C&C Generals, GTA), thats it. funny thing is i did a visual inspect of the chip about one week ago and have detected a physical hole, completely through the chip. it looks like it's melted. totally ridiculous. i'm pretty fed-up over it, i think i'm just gonna buy a dell next time. rather amazing though is that it does still work. can i post a picture of it on here? maybe i'll put the pic in my sig file. regards and thanks to ALL posters here, this is a wonderful forum.