Intermittent sounds like either a bad switch or a dry/cracked joint. So it may be worth taking it apart, removing the PCB and looking for bad solder connections. Most of the components will be surface mount components on the top side of the board, but switches and pots will be through-mount components with solder connections on the bottom of the board, and so fairly obvious.

I've just noticed that the digital FX are on a piggyback board, so it may be worth making sure that the connectors are making properly. Remove it and push it beck in full, just in case. Unlikely, but you never know.

And as there is no footswitch being used, it maybe that the footswitch jack contacts themselves are getting corroded through not being used. As the channel selection signals from the front panel switches are probably passing through the jack socket, I'd spray some contact cleaner in the footswitch jack and push a jack in and out (any type of jack) several times, to get the contacts making and breaking. This is often a problem with series FX loop jacks that don't get used on an amp. Ideally done with the PCB exposed so you can positively spray claner onto the switching contacts, but spraying through the hole with the board in situ is better than nothing.