Speed control

A strange question I know but I have a lure machine, racing dogs chase the lure, made from a car starter motor. This is controlled for speed by switching a solenoid on and off with a push button . This is ok but the motion is jerky and hard to control in front of the dogs . What I would like to do is fit something that would control the speed more accurately. Any ideas . The motor is 12V and 0.8 Kw . Thanks