My first guess is the loop section is randomly going into oscillation. Is there any sign the previous owner has "adjusted" any of the tuning coils?? Next time the light wont extinguish, carefully try tuning one coil off resonance to see if effects the symptoms. If so, it is reasonably easy to retune these correctly. These coils are in a row on the right? hand side of the RF board.? The first signal IF are the coils on the left. Also check for any damaged components in that area,? especially bypass capacitors. This stage operates at around 55 MHz with very high gain.
