On most receivers, check the switches, and clean their contacts. Many switches can get electrically dirty, the source selector switch, the tone switches and tone defeat switches, the speaker select switches. A lot of receivers have preamp out and power amp in jacks in back. Some require a jumper plug across them, others defeat those jacks with a switch. Put that on your list. Any tape monitor or recording monitor switches could also be involved.
If flicking any switch up and down or back and forth makes the left one come back up, even for a moment, that points at the switch being dirty.
VOlume controls are often dual units, and the balance control could also be involved, so rotate those around to see if they seem dirty.
Of course it could be a real electrical problem, but my experience is that these dirty contact situations are quite common.