This sounds like another case of bad connections or soldering in the amp. If it's not too old take it back to your dealer, they should fix it. These are designed (allegedly) to be used and moved about and should not fail. If it is too old then open it up and with care - and with yourself isolated from earth and wearing insulating gloves wobble connections and components until you find the cause. We have had a couple of this sort of fault with newer mackie stuff and it's not been too hard to track down and fix if you're handy with a soldering iron.

If you have any doubts about being competent to do this sort of thing take it to someone who knows what they are doing, it should be a fairly quick fix.