You didn't say if it was tube or SS, but I'll assume SS because of the vintage and the obviousness of a tube problem. Capacitor or resistor (heating/cooling makes for auto_intermittancy)is a good possibility or oxidized solder joint. You might check whether physical pressure or tapping on the board (assuming SS and cicuit board) has any affect. If so, this could bring bad solder joint to the top of the list.
