Entire machine will vibrate excessively while lowering the primary lift cylinder with the hydraulic control lever at half stroke, and a full load applied to the forks. We have already repacked the primary lift cylinder, gone through and inspected various hydraulic valves checking for internally broken springs in valve bodies, installed a synthetic hydraulic oil in place of the regular hydraulic oil. Problem still exists. Anyone have any ideas what could be causing this? If you have any suggestions, please let me know.