Yes, "running rabbits" because of the way the spots would often race away from the scope centre.
If two otherwise-identical transmitting and receiving antennas are rotating at slightly different but constant rates then (all else being equal) the interference "blip" will progress in or out along some set of trace radii at constant range rate, as Brad says (in or out depending on which rotation is the faster).
But if the relative rates vary then the range rate of the blip will vary. So if the transmitter's scan rate starts out being slightly less than the receiver's the blip will move in, if the rates are equalised the blip will stop, and if the transmitter rate then falls below the receiver's the blip will head back out again.
Bear in mind that these are tiny differences, just microsecond asynchrony, and we're talking about electrical rotation of heavy machinery, so this sort of thing will tend to happen quite often if the turning gear of one or both radars is only slightly imperfect and has a cyclical drift in rotation rate.