Hello!
I have almost exactly the same problem with a unit I've tried to repair for a guy who lives nearby me in Sweden.
Read:
http://forum.grailtone.com/viewtopic.php?t=16493
After all that I emailed "fatboy135" who gave me tons of advise.
(Thank you fatboy)
I've checked all supplyvoltages for the whole amp including the digital board, cleaned all the ribbon cables and checked connections, and (believe it or not) replaced every single ic (including 8mhz crystal) in the digital circuit, and still nothing has changed.
The problem lies somewhere else, but where?
This is not good news for you of course, but on the other hand this info might help you see what i have overlooked.
If you or anybody else comes up with a suggestion or solution, please post it.
(Sending it halfway around the planet to Mesa is no option for the guy, buying another used unit would be cheaper, or less expensive).
Regards
Mikael