Help required: Problem with studio light


The above image with the dark band happened a few times when I last used my studio light set up.
D200 PC cord to 1 light the other working as a slave. Manual exposure and F Stop. SLS4000 strobes. Worked into silver umbrellas. Shot onto a a muslin backdrop. People approx 2 feet away from the backdrop. Strobes to the left and right of the camera.
I think a mode on the camera might be incorrect but can't work out which one.
I have another shoot tomorrow night and would like some input as to the possible problem.
The lights were fully charged ect.....
All input appreciated.