Intel 845G operating temperature?

Not sure if I've picked the right forum. I'm trying to repair a friend's Dell Dimension 2400, which uses an Intel 845G northbridge. The computer would shut itself off after a few hours, or in as little as 20 minutes. The fans still ran, but all the computer functions would go, and (as Dells do) the tower would signal an amber power LED. After eliminating most other possibilities, I checked the northbridge and found a brown film on the substrate, apparently from the thermal pad smoldering. I've since replaced the pad with thermal paste and attached an external LED temperature probe to the substrate (cheap Dell = no BIOS/software monitoring). It's showing an average of about 49.5 C, which still seems hot to me. (OTOH, it's been running for only about an hour since the operation...) Can anyone tell me what the normal temperature is supposed to be? I've found a couple Intel PDF's, but these seem to list only the maximum temperature. Thanks!