You don't say which process you aree using but I'm curious as to whether you see the banding printed out or if it only becomes apparent in the processing trays. I've had films band when using other than flat-bottomed trays. Perhaps some alt prints could suffer from the same problem.

If it is a banding caused by the "lightbulbs" (UV fluorescent tubes or ???) , you could also try moving the tubes closer together instead of raising them relative to the printing frame.