Not such a long shot, perhaps -- good thinking tmtoh! Pots regularly develop intermittent behavior (if you've experienced a noisy volume control pot, you know what I mean). And it's easy to test for, so Flamingtoasters should do so before tearing into the guts of the thing.

First note the position of the adjustment (so that you can get back to it later). Then jiggle it, rotate it a bit, whatever and see if that has an effect. If it does, then verify that it's not a bad solder connection to the board. If it is a noisy pot, post back.