[PhilcoMike]

Ron,

I don't mean this to be a step by step tutorial but a few suggestions having been through similar problems. In older receivers like this one your fingers or a IR thermometer can be your friend. When a channel is biased wrong is will run hotter, especially the later stages output transistors.

Another area to check out is some type of thermal-component issue. While transistor testers are good they sometimes don't pick up on thermal stresses. If care is used a hair dryer and a can of cool spray can be your friend. By thermally heating and cooling some areas you can narrow down the search. I had a speaker protection transistor in a JVC that gave me fits. The only way I found the problem was alternating heat/cool until I hit this transistor. Bingo!

Your description of fine and then distortion returning sounds thermally related. Assuming that voltages are staying stable.

Good luck,

Mike