Sounds like the storm we had here the other day has reached you!
I am still trying to picture it running and cant see a reason that it would be that jerky! ---- Try just cracking the valves a bit and see if it still jerks. Regular valves are not motor valves----people use them but I wouldnt. Motor valves dont jerk---they freewheel when released and regular valves lock up making the motor grabby.
Not sure if there is a special valve that you could install between regular valves and motors or not that would soften the blow.---probably not but just a thought.
