The pots (potentiometers) are dirty.
1. Get a can of deox it or other electronics cleaner.
2. Unplug amp!
3. Get inside of amp and check on the back side of pots. If there is a small hole
spray some cleaner in there and rotate knob back and forth frpm 0 -10 several times.
If there is no hole, Remove knobs and spray down the shaft in the front give it a minute to get down in there then rotate knob several times. do this to every pot.
4. Put back together and Jam