Almost certainly a physical intermittent, nothing to do with the microprocessor, or settings. As someone suggested, a bad solder joint or a relay. If you're comfortable with it, pop the cover and wiggle things around. Aside from the raw 120VAC probably nothing that could hurt you. Use an insulated tool just to be safe.
Also, you could clean and exercise all the switches and controls, with contact cleaner. And any chips in sockets, give them a push down with your fingers. You might hear a crunching sound as the contacts shift.
In addition to wiggling things, you could try circuit cooling spray on individual components.
Intermittents can be a bear to troubleshoot. You have to be comfortable with diddling around with things while it's operating. Unlike tube gear or ESL innards, there's very little inside that can hurt you.