Clemens,

Glad to hear that the firmware upgrade worked. It is a stressful experience!

I'm looking into intermittent banding issues as well. My banding is visible in deep shadows only.
First I thought it was a calibration problem, but the bands are never in the same place. I'm fairly convinced now that the problem is related to voltage variations at the light bulb, which in turn could depend on external voltage variations or internal variations. Aztek recommends a voltage regulator. The other thing to do is to clean and reseat all connectors and circuit boards.