I'd send it to Mesa...
I just talked to Chris at Mesa yesterday. They are going to repair my Mark II C+. It has some intermittent vibration (fractured solder joints I'm sure) issues that I have methodically chased for months and cant seem to locate. Mesa's are cool amps but tend to be dense in construction and a pain in backside to work on.
They will run the amp through a "period correct" factory check-out. That will usually expose issues and they will correct them.
He said most times it is less than $250 since they have the know-how and have likely experienced the issue you are having and can fix it quickly. Look on their website and call the number. You'll likely have to leave a message, but they will call you back eventually.