Here is some additional info on Monolights and flash duration here:http://jerrycentral.com/2008/12/18/flash-duration/
As Neuro... said... the higher the power the shorter the flash duration for Mono's.. So as one may have suggested... backing the lights up and going to a higher power might be a fix.
I can easily introduce blur with a non-IS lens all the way up to shutter speeds of 1/1250th of a second just by panning slowly at 70mm. So I can see where it would be possible to get some motion blur even with a monolight if you introduce a lot of shaking when you push the shutter. So possibly you could work on your technique. I know that I used to have a jerking issue when I pushed the shutter, but have resolved that issue with practice. Otherwise... a "Steady" tripod should also resolve the issue and also a shutter release should make it perfect.
If you look here:http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=108&Camera=453&Sample=0&FLI=0&API=6&LensComp=108&CameraComp=453&SampleComp=0&FLIComp=0&APIComp=4
it looks like this lens is sharpest around 5.6. I wouldn't go above F8 unless you really needed the additional DOF.