Thanks, I'll keep checking with your progress. I wish I could be more helpful.

If it's a problem with the CPU, maybe you could look towards Adrian's Digital Basement on youtube for advise? He seems to diagnose and repair vintage 8-bit computers with similar issues and ends up replacing chips on a regular basis, even just figuring out how to diagnose defective chips might be helpful in this case. I just remember he had defective CPUs in the past that he needed to replace. I think he has a website with contact information as well.