Have you checked your fuel pump?
I have seen fuel pumps fail on 80s that cause very low power and stumble at low rpms, sometimes under 3k. I have also seen fuel pumps fail that cause a failure at high rpm. It may be on its way out and causing...
I wouldn't want to scare you, but that radio lasted for about two weeks on my car. For less than 20 bucks it was worth the try, but it didntl ast sadly.