Hi there. Due to your diagnosing you can be sure the problem lies with the controller board. Did you try to unseat the processor and re-seat it ? I read once that solved some problems.

Personally I also would do the same with all other IC's that are not soldered to the board and on the ones that are look for bad solder points.

As it reversed suddenly I would think it is in one of the IC's and not any other component.

Does this happen immediately after turning the machine on ??