I've used the 6000+ before and they aren't well known for their reliability however if I were to wager a guess I would say that one of the
SCR's is burnt out. Usually each SCR controls 2 channels, and they are what do the actual dimming. What would suggest is opening the unit up and checking to make sure there are no loose control wires or any loose wires at all and if that doesn't work then I would replace the SCR. SCR's are pretty easy to find just need to know where to look. Usually they are also easy to replace since they are just screwed onto the case and the wires connect via crimp connectors.
Hope this helps!