I'm not sure I have any more troubleshooting specifics than you will 🖖🏻. And my audio equipment experience is limited. But here's my two cents from what I know about circuit boards and audio. [br]
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Something is probably generating interference or putting electrical noise into the system. It could literally just be that everything needs a good cleaning, something has some grit stuck in it, or some cable is touching something it shouldn't. See if you can find something that triggers a specific behavior, or makes it worse.[br]
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Give all the internal bits a good once over. I trust my eyes before anything else when it comes to finding problems. Add some pictures if you want extra sets of eyes. And keep us posted because now I'm invested.