I saw your post, but I didn't want to give you bad advice and make things worse. It's been awhile since I worked on a Williams System 3-7 board. I thought surely someone more capable would help you.

*edit* Nevermind, I see where you said you did reflow the solder on the connector pins. My fault for not reading closer.*/edit*

Replacing the rotten Scanbe sockets is usually a good idea. Some people say the pads and traces for them are too delicate, but I've never had a problem.

Even if the sockets and cold solder joints aren't the issue, it's good to eliminate them first before getting into the heavier troubleshooting with test ROMs and logic probes.

If you can just pull ICs right out of the socket without much effort, then your socket is probably worn out. As to whether it's a Scanbe or not, the name is stamped on the socket body itself. They look like this:

scanbesockets.jpg

One reason they're notoriously failure prone is the weird way they grab the pin legs. Instead of gripping it across the flat edges of the pin, it grips the side of the pin.

The top three empty sockets on your MPU board don't seem to be Scanbe sockets, since they're open body sockets.