It sounds like a bad solder joint or a loose connector. If you are comfortable removing the amp from the case, first look for any lose connectors (AC unplugged). If none are found then plug in the amp into AC and use a plastic handle of a screw driver (or other plastic device) to tap around the circuit board. With the volume set a a low setting, the place that makes the loudest static with light taping is usually close to the problem area. Unplug the amp from AC and carefully look for a bad solder joint. You will need a soldering iron and solder to fix.