To me, it sounds like a poor connection to the motor. That is what a D.C. motor will do when it is not getting its full voltage, or is starved for current, because that's what Ohm's law would have it do in that situation. I would check both the power and ground sides of the circuit. Maybe start by putting a voltmeter across the motor and see what the voltage does as the motor surges. If it stays constant, it's probably a motor problem, while if it goes up and down with the motor surge, it's a power problem.

OR - maybe a weak battery causes the slow running (esp. if there is other current drag on the system, such as lights), and the surging is caused by the voltage regulator kicking the alternator into "charge" mode?