Well all seemed fine, I was working on Kermit, making good progress, then I/O PARITY ERROR AT xxxx -- disk read error. Diags etc, another shows up. Then another, but a previous goes away... Clearly there is a soft(ish) read error. Probably all those cheap ceramic disks on the read/write/amp board. So I will order nice, new, correct monolithic caps, and replace all the replacements. It's almost certainly in the analog read/write section, and not hard logic, said hypothesis boosted by the fact of my fiddling in that arena. I ran memory diags and all that stuff, but I am assuming it's the electronics I fiddled. I'll go do the job Right.