[hsusmann]

IMHO, it sounds like if the radio plays for 5 seconds then goes dead
it is a capacitor charging up somewhere.

Is it possible that there is a missing or open resistor on a grid
somewhere that lets the grid end of a coupling capacitor float upwards?

Perhaps measuring some grid voltages in the audio section will
turn up something? This can be tricky, though, as sometimes
the meter will pull the line back down to where it is supposed to be.
If the radio starts playing again, however, then this is the problem.

I had this happen once on a radio long ago.

Herb S.