Rotel RA-870 intermittent channel (simple fix?)

[Trintignant]

First post here, and so predictably newbish. I got this Rotel integrated amplifier real cheep as a second unit and worked perfectly for years alongside my Creek. I've never really maintained it, but all the pots and switches have always been smooth and clean-sounding ... But recently the left channel has been cutting out.

Often after I play from a steady source for a long time (tuner, say) and don't touch anything the channel will come back and sound great, but then if you so much as breathe on the deck it disappears again (fiddling with any switches: stereo/mono, input, mm/mc pretty much guarantees the channel to drop -- it also happens from vibrations and whatnot; the volume control has nothing to do with it, though. The headphone jack is unswitched).

I can post pics of the inside if that would help. I don't know much about stereo electronics specifically but I know electrical parts OK—can solder and all that (though have neither iron nor multimeter right now)—and as far as I can tell the only dodgy connections visible from the top of the board are for LED diodes, all of which work fine.

Any theories where I should look for or what I should start by cleaning?

Thanks, guys.