One other thing to try is to swap out all the actinics w/the 10K ones just to make sure you don't have a bad bulb or two.
Is it a new fixture? if so get it repaired/replaced under warranty. If you bought it used, unplug it and check for loose wiring and if that looks good then you could try swapping the ballasts themselves. If the problem travels to the other lights as I bet it will, then you can go ahead and order a new ballast. If the problem does not travel, then the switch could be bad.
Safety note: don't mess with the electrics unless you have experience doing so. It's not that hard, if you do have that experience, but miswiring could be dangerous. Don't take it apart of mess with it if it is new and under warranty-that will be a problem if you have to return it to the manufacturer.