Has it always done this or did it just start doing it recently?
Hydraulics moving like that, or a jerking motion, or even slow behavior are generally because of low fluid level. Also check for obvious leaks. A plugged suction screen or return filter would affect performance too. So I would check the easy things first ... leak and fluid level and clean the filters. Symptoms are more apparent at low roms than at high rpms. Once the fluid level is correct, and your filters are clean, try it again.
If the problem persists, then it could be worn "O" rings or glands in the hydraulic 3pt cylinder.
Another thing to check is your position control lever. Is the lever moving (creeping down) while you are working the tractor? It may need to be tightened a bit to stay in the position you put it?