Hello. My name is***** can help you with your question.
The first thing I would try is changing that potentiometer that is turning with the bass knob. It's possible that it's simply gone dirty or that it's failing and that will cause what you describe. You don't necessarily have to be moving the potentiometer for this to happen. But if it is set it's to an area that is damaged, you will get some very bad results like you explain.
Let me knowIf this answers your question or if you need additional information or troubleshooting.