It is an interlacing issue. Try rendering (codec settings) it as progressive first with blend fields then with interpolate fields. See which one works best. You might need to change the project properties too.
Mike K is great at these questions he knows it way better than me, maybe we’ll hear from him.
Jeff
2-Xeon X5680 @ 3.33, EVGA SR-2 Mobo, 48GB DDR3, GTX 580 3072MB, 16TB Attached Storage, Win7, Vegas 11 x64