h120ft sn N005V02614E

this truck shakes really bad when you hit the lift slowly. truck has fingertip controls. if you pull the lever back normally the truck lifts normally. it only shakes when you barely pull back on the lever like when you are trying to lift very slow. i cannot get into the dash as the password has been changed from the factory password. i did try to swap the lift and tilt fingertip control but it had no effect. my salesman said the guy he bought it from had it diagnosed previously and they told him the mast was leaking through the poppet valve. it does seem to drift slightly after you first let off the lever but not that bad so im not sure if that has anything to do with the problem at all. anyone have any ideas of what this could be. im guessing something in the control valve but that is a complete guess. thanks in advance for any help