I bet I know what it is... there should be some kinda weight attached to one of the driveshafts (the long one I think IIRC), looks like a lump that goes around the shaft, at the wheel end. If it's missing, it can cause this if you settle to a speed. And it can be any speed, not just 70... I got shown it by a mechanic ages ago, and I thought he was mad at the time.......!