Jady,
Before sanding (or even reseating), I suggesting do as much as possible a check on the power train to/from the rectifier board, SD board, and MPU. I think it will either come down to a faulty MPU (either faulty board component or loose chip connection) and/or bad connection of power in between the boards.
I strongly suspect you will find a power issue (and an intermittent one at that). I would try testing immediately after a failure, and leave power on (ie. don't turn anything off). Of course, you will need to be careful checking with the system live. Test all of the test points on the boards. Assuming it is a power train issue, and also that the power doesn't "correct itself" immediately after the failure (which may happen if the issue is an intermittent/touchy connector issue), you should be able to test the power trails through the various connectors.
Displays going blank. Maybe a starting point. If this happens again, test for the HV voltage on the display boards. If the HV is not there, likely nothing to do with the MPU, but very likely the SD and/or rectifier.
If you need the schematis for Skateball, I have them, but you can also easily get them off the ipdb. I had an issue like this with my Bally Freedom, game would play, then stop suddenly. Ended up being a power train issue, and nothing to do with the MPU. But it took a good couple of hours slowly but thoroughly going through the power train between rectifier, SD, and MPU to confirm.
