IF you can get the thing to fail while out of the case, the quickest way to find a thermal failure is with circuit cooler spray. Unless you are adept at electronics, this is probably NOT a DIY project.
The nature of your symptoms SUGGEST probably a bad solder joint or a cracked circuit board.
OFTEN the bad solder is around HEAVY components or POTENTIOMETER leads, especially where they support the board by those leads (dumb!)
A trained eye can spot bad soldering around a lead... you look for a tiny gray line around the lead part way up the meniscus of the solder fillet... see one, reheat the joint with a soldering iron.
Answers & Comments
IF you can get the thing to fail while out of the case, the quickest way to find a thermal failure is with circuit cooler spray. Unless you are adept at electronics, this is probably NOT a DIY project.
The nature of your symptoms SUGGEST probably a bad solder joint or a cracked circuit board.
OFTEN the bad solder is around HEAVY components or POTENTIOMETER leads, especially where they support the board by those leads (dumb!)
A trained eye can spot bad soldering around a lead... you look for a tiny gray line around the lead part way up the meniscus of the solder fillet... see one, reheat the joint with a soldering iron.