After you guys helped me to understand the speed-induced stripes in my images, I have a new problem: evry second sweep seems to be offset by a small ammount. Basically the bottom line is a zig-zag, see attached. It has happened in all recordings from today and none before. It seems to be originating from within the unit, as it really is always precisely every second sweep, so no waves, interference etc. Any suggestions? It looks the same in humviewer and reefmaster, so not readout problem. On the screen I didn't see the problem, but I might have overlooked it...
Edit: I found exactly the moment when they kicked in, see image 3.