Thats usually a tell-tale sign of air in the system, o-rings on both ends of the piping coming up to the pump or the shaft seal in the pump itself. With engine running slow, loosen the bleed plug on top of pump just until oil starts coming out (not too much or it will blow the plug out and you'll get a hyd. oil bath) let it seep for a few seconds, tighten and see if the lift works smoother.