Your condition is called the hiccups, and it's caused by an internal leak, which could be caused by any number of different reasons.
To help isolate, raise the heaviest implement you have to full height and shut the engine down. Time how long it takes to either fall to the ground or else fall "x" distance. Start the engine, raise it back up, and pull out the ASC valve (black knob on the RH side of the lift cover). Shut engine off and repeat the test. Report back here with your results.
