[Arran]

As I mentioned I have encountered problems with the solder joints on grid caps, especially on the older tubes, they seem to oxidize causing open circuits, or high resistance, and heat or vibration can cause it to come and go, the cement holding them on often fails too. Though what you are describing also sounds like the oscillator stopping, it could be the coil, a resistor, or a bad mica cap. One idea I have heard of as well was to bake the coil to drive the moisture out, though soaking it in a pot of hot wax will do the same. These autodyne oscillators were notoriously unreelable, I've been looking at an Echophone S-5 and the early version used a single #24A as a mixer oscillator, in the later ones they added an eighth tube, a #27 as a stand alone oscillator because they were having so many reliability issues. My guess is that with Philco model 89 sets it was a royalty avoidance scheme since they used the same ruddy front end for four years, even though the model 60s started around the same time and it had a 6A7 pentagrid convertor.
Regards
Arran