If a potentiometer goes bad over time, it can cause the sort of behavior you described. If you're still within the first year (from the date of new product purchase receipt), you can get the amp looked at under warranty.

If you after that first year warranty period, you can purchase replacement parts from any of our Authorized Service Centers, but here's one that sells parts online and parts can be shipped (in the domestic U.S., that is): http://avalancheproaudio.com/2801.html

Most like it's the potentiometer for the Amp Model knob, but a tech could verify it in person.