I am an electronics tech by trade and one of my jobs from about 1980 to 2000 was repairing consumer electronics, including car stereos (lots of car stereos... GM, Ford, and aftermarket). I have seem many times intermittent problems such as this would end up being some poor solder connections in the unit itself. The radio would have to be pulled from the car and opened up on "the bench" to repair. Sometimes the connections are a little hard to see especially if there is conformal coating (similar to a clear coat to help ward off issues from condensation) on the circuit board. Keep in mind with the hair line crack on the solder connection, the inner part of the connection will contract/expand at a different rate than the outer part of the connection due to different amounts of solder and copper in that area of the board. This results in a poor connection which can randomly work fine when cold and cuts out when warm (of course the opposite can occur too... bad when cold, good when hot).