The 53C80 SCSI Controller chip (UI12) can go bad, and when it does, it gives you this symptom. Luckily, you can buy replacements for under $10 apiece, but you'll need the proper hot air rework solder/desolder workstation to swap out the bad part. I don't have one of those workstations and am diligently trying to wire up the chip with enamel coated wire running from each pad to their respective leads on the IC. A pain, but am making progress; I'll keep everyone posted and with pics as to how it turns out.

Apparently, silicon does go bad after so many years and no amount of recapping can fix the problem. It may be even more serious enough to spread to the other components on the top or bottom of the logic board. But this depends on storage environment, how often it's powered up, age of the Mac, etc.

Another item to try replacing is the Bourns network filters (RP2, RP3 and RP10), maybe check the fuses (F3 & F2), diode (D3), Serial Controller Chip 8530 (UG12), not to mention the various resistors, capacitors, diodes and inductors on the underside of the logic board. So I'm talking that in some cases, you may have to go as far as totally rebuilding your logic board to fix an issue like this, but in other cases you may just have to replace UI12.

Then again, it could be as simple as bad data cables or termination that isn't right.

73s de Phreakout. :rambo: