Sounds like garbage on the data or address bus. An easier solution before you pull out the soldering iron again. Is to either replace the CPUs (the chips themselves not the board) with known good ones, or use a logic probe to poke around the pins to see if any critical ones aren't strobing. Preferably the logic probe first along with voltage checks with a meter to make sure nothings wrong that's going to also destroy your good CPUs. It could be a PIA too, which you can also do the same procedure too. But really once you start getting this deep, unless you really know what you're doing, it's probably best to just buy a new after market board.