I GUESS it could be a component that fails when it heats up. I only know of 2 persons to whom I would send a board for repair. One is MTH in Columbia; the other is GGG, based on his reputation.
Given that it is apparently an old 5-volt system, I'd probably gut the loco and put in one of the 3-volt upgrade kits I have on the shelf or a new PS3 kit. If I didn't consider myself capable of doing the switch, I'd have GGG replace the board with a PS3/2 board. That is a job that several techs who post would be qualified to do, but not all.