The problem is likely either a cold solder joint somewhere on the printed circuit board or one of the jacks.

The next time the sound fades out run a short cord between the preamp out and power amp in jacks that are on the front panel.
If the sound comes back right away, then the power amp input jack is likely the problem.
It may have a bad solder connection to the board, or it may need to be cleaned or replaced.

Either way, be careful if and when you take it apart. Those tended to be fragile amps.

Steve