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1/250 makes the photo cut in half. half the photo is a photo the other half is all black.
Yep, you've exceeded the sync speed of your camera.
The D700 can sync up to 1/250, but third-party lights, especially larger strobes, and/or radio triggers with their propogation delay, can cause slower sync.
...125 works well though. so 125 f11 and iso 200 seems to be good.
If there's little or no continuous light on the subject you'll find that 1/125 @ f/11 and 200 ISO, 1/60 @ f/11 and 200 ISO, and 1/30 @ f/11 and 200 ISO all look the same exposure-wise. As I said above, as long as it's within the sync speed of your camera, shutter speed has no effect on the flash; only on the ambient light. A 1/1000-second (or there about) burst of flash will give the same amount of lighting on the subject at all the above shutter speeds.
Slower shutter speeds would let more of the ambient light record, though, which may or may not be a good thing; only you can decide, based on your shooting conditions.