[MrFixr55]

OK, from the crazy idea department:

If you have a signal generator that can be externally modulated, hook up a pleasant audio source to the generator, pull the 6A8 and connect the generator to the IF stage. Listen to the radio for a long time to determine if the issue occurs. If you play the radio for a very long time (days) and the issue does not recur, you can possibly rule out the IF, Detector, AF stages and the power supply and concentrate on the convertor stage. Set the radio for the highest broadcast band frequency (Tuning cap fully open), couple the signal generator to the antenna and tune the signal generator toll the signal is heard. Play for several hours (days?) If the issue does not occur, check the tuning cap. The aluminum plates of a cap corrode and create "whiskers" similar to the stalactites and stalagmites in a cave. These can short out the RF stage section of the tuning cap, attenuating the signal.

You can use a set of feeler gauges to clean the gaps between the plates of the tuning cap.