Uppo's on it, rail alignment-jack alignment-dry rails, the list could go on and on. Roped hydros can be an absolute pain. You could have a tight packing, the jerking motion of the piston traveleing down out of the top landing could cause the safeties to set. Tight packings will be worse the higher the elevator travels, you may not notice it too much in the lower portion of the hoistway, the higher you get the harder the "slip, stop, bump".

The correct oil would also make a difference, if the oil has overheated-maybe only once, it will need to be changed. 155F degrees is the cut off point for hydro oil, if it has ever risen above this magic point in temperature, the oil, valve, packing and and any other peice of rubber in the oil line will need to be replaced. Oil is acidic in nature, polymers are blended into the oil to prevent this acid from eating components in the oil system. This acidic oil will now cause many issues with the proper operation of the valve and packing. Acidic oil will turn most rubber "O" rings and packings into sponges, instead of sealing, the rubber now soaks and swells up creating many problems.