HELP - right exposure time for strobe

Hi all, need some advice on why this happens to my shots.

Apparently, anything above 1/320 exposure time will result in the black "band" in the photos, I suppose due to the sync speed between the camera shuttle and the strobe's ability to fire?

Am shooting D700 with 60mm in Aquatica with 2 x S&S 110a.

Many thanks.

Sample shots:-

Here is info from the introduction announcement for the D700.

A top shutter speed of 1/8000 and a top standard flash sync of 1/250 (or 1/320 via a Custom Setting), plus 1/8000 with a compatible Nikon Speedlight and Auto FP mode enabled.