If your amp has insert FX, just try a little contact cleaner on a 1/4" jack and plug in/out a whole bunch of times(like sex...
) to clean your FX loop, which is usually "in line of the circuit", which has caused many an amp of mine to seem like it had problems and it was just losing continuity at this point, and very erratic. One day it's fine, then drops off til you clean them, maybe returns, etc, until you get in the habit of doing it just before playing, plug in/out of your FX loop(even without cleaner), and it will remain clean and well connected. Hope this helps.
Anybody have any tricks on removing the boards from the chassis on a VS 8100 head without desoldering a bunch of wires/transformer, etc.(see prior posts)?