I did a good check with magnifying glass and even traced a good chunk of connections using schematics found at this forum but everything checked out. So I see why everyone says cold solder joints and traces are rarely a problem. I have two working 512ks somewhere in my pile. When I get some free time il have to try and use one of them as a donor board.

Teknight, I can't think of anyone else more qualified to get to the bottom of this. But I'm curious what your troubleshooting process is. Do you get a jig extension and power the board up while probing every chip for expected logic output? Is an oscilloscope your best bet, or perhaps a logic analyser? Or desolder one chip at a time and put it on a breadboard to test it's full functionality against its data sheet/logic truth table. If only that equipment was not so expensive I would think it fun to get to the bottom of the issue.

Some IC somewhere in these boards has gotten nasty bit rot I say!