Generally, I'm thinking:
If signal goes up'n'down non-rhythmically, it's likely related to bad connections, soldering..
If signal does it rhythmically, it's likely caused by an oscillating circuitry, involving a capacitor.
If [dialed-in]distortion/gain remains constant, it likely isn't in the tube gain-staging section.
I have a problem seeing this problem located with the main volume vactrol; even though this one also does the tremolo effect, this is controlled by an algorithm in the software and I cannot see how such an code snippet can become unstable.
Over time, we've seen several issues with the compressor circuitry.
When this signal gain wobbling problems is present, is the compressor set active?
Another potential: When touching the various inst/line-level switches on the back panel, does gain stay constant, or does is result in flickering gain + scratching noises?