Nice looking tractor you have there. What you are experiencing is what is called "hick-ups". It is caused by an internal leak somewhere in the system. Could be you need to replace the piston and "O" rings for the lift cylinder, or some other internal parts.
If your vain pump was replace with a piston pump, be sure there is clearance behind the pump and the oil line fitting behind the pump. If there is no clearance, then you need to grind a bit off the back of the pump where it is touching. Failure to make clearance can and will break the ears off the pump mounting flange. Don't aks me how I know this. My piston pump now has been welded or brazed after clearance has been ground. Sometime I have to learn the hard way. Hope you won't.