Jerky points towards pump not getting oil.
First things I would check are:
hydraulic oil level,
oil condition, I.e. is it milky colored from moisture ingression or foamy from air?
Is there a suction screen on this model? If yes check to see if it is clean?
Is there a inlet filter? If yes when was it changed last?
Where is the pump mounted? If any external inlet lines double check to make sure that none of the rubber hoses are cracked / weather checked. Check all of the connections to make sure that something has not worked lose over time.
