I know this is a silly question, but, if your willing to spend money on an amp tech, why not just remove the amp from the cabinet and send it back to Mesa? Maybe they know something we don't.
Try dipping a 1/4 inch cable tip into contact cleaner and then insert and remove it several times from the effects loop inputs (send/return). If these contacts get dirty the foreign matter can hold the contacts open and can intermittently kill the signal...
You did the right thing, and I hate to say it, but one day you will have the bittersweet moment of getting it back, and you'll always keep it because it was your Dads.