Sounds like it "amps" out. When amperage gets to some point because it is binding it stops til it cools then will go again. Can also be low voltage. Especially on hot days it is worse. Low voltage can be caused bu too small wire supplying the motor, a bad splice in the wiring someplace.if you hook up 12 volts directly to the motor and bypass the controller and it goes in and out with no problem start looking at controls or wiring. I hab same problem and it took 5 years to figure out then find a bad splice between the controller and motoe. Only splice was by passenger seat. That was it.