Cub Cadet Walk Behind Surges

I have a 2008 self propelled cub cadet model # 12AE18JA056 with a 173CC motor, and auto choke and throttle. It surges constantly. I cleaned the carburetor, fuel lines, and tank thoroughly. Changed the oil and spark plug. Tightened and un-tightened the springs. Nothing works. When I manually push on the lever attached to the governor, it idles perfectly. Conversely, when I pull it, it revs high and smooth, but as soon as I release the lever, it does the same thing- surging up and down. Help!