I found some discussions on the Net that lead me to look under Settings / Applications / Battery use to find that an application called "Media" was overusing the processor and causing the overheating and drainage, so I force-stopped that one and the problem went away.
But now every time I connect the phone to a PC in USB mass storage mode, I get the same problem and after I'm done transfering files I have to force-stop the "Media" application. (This is even if I use Safe Disconnect or whatever it's called on the PC.)
I've seen some other suggestion today that I should try the same operations with the SD card pulled out, but the same thing happens even in the absence of the SD card.
Does someone know how I can reset/reinstall the "Media" application or service that handles USB mass storage mode without resetting the whole thing to factory settings or reinstalling the whole Minimoto image?