Zapper,
Maybe Brian might have an idea, but sounds like a driver failing, or an intermittent connetion
If you suspect the pots then try this:
1) Get a small screw driver that fits the pot shaft.
2) Make note of where your setting is
3) Then "gently" turn the pot clockwise and counter clockwise as fare as it will go in each direction, about 20-30 times
4) Then reset the pot levels, and see it that helps.
Sometimes the wipers need to be "cleaned off".
Also check all you speaker wire connections and make sure they're good and snug.