This is a long shot, but...

I had a Mystic CC once where the red would vary from time to time. It turned out that the red adjustment pot - the red adjustment that's one of the series of little holes at the back of the analog board - was really loose from age or wear. Its adjustment was very hair-trigger, and it seemed that the slightest movement of the machine, or perhaps heating and cooling inside the machine with normal use, would make it shift ever so slight and change the red balance significantly.

Best,

Matt