Understand that I have no experience with your particular amp...

It seems to me that whatever handles the channel switching is not fully switching. If its done with mechanical relay(s), there may be some intermittent problem with some of the contacts. If the switching is done with something solid-state like JFET's, there may be some leakage in the control circuit that keeps the channels partially open sometimes.

My guess.

For the real scoop, you need to take it in to a tech who knows this particular model of amp.