Well, I've had it apart and discovered that there is no thermal grease on the 603 chip. No sign of any at all. Clean as a whistle. I can't see how there could have been any applied at manufacture. What there is is a mechanical arrangement that applies pressure to keep the heatsink in contact with the processor.

More importantly, I discovered that the area that is getting especially hot is to the left of the CPU, and a little further from the whole CPU area (as in, 2" or so) than I had thought. The area in question has a concentration of several largish capacitors. So maybe it's the infernal capacitor problem once more.

There is no obvious sign of leakage, but as I have replacement caps with the required ratings, I think I will replace the capacitors, clean the logic board up, and see what happens. I can't make it much worse, can I?

Overheating is indeed symptomatic of capacitor failure, is it not?