A few quick questions:

Is/are there relays involved? And do they 'click' when this problem occurs?
Are there any other symptoms?

Yes. Clean all controls and switches.
Yes. Look for loose boards and loose connectors and looms from board-to-board.
With the top off, and with a wooden stick such as a chop-stick or drum-stick, poke and prod (gently) here and there - systematically - from one end to the other to see if you can induce the symptoms. Pay special attention to the outputs.

I would also look for swollen, burnt, or leaky electrolytic capacitors, evidence of burnt parts or other visible symptoms. If there are relays, those would be the first thing I tapped.

This does, superficially, appear to be a mechanical issue and/or a power-supply issue if the relays are not receiving enough current to latch (not uncommon).

Good luck with it!

Peter Wieck
Melrose Park, PA