Sounds like a leaky lift cylinder seal. The control handle linkage is made to keep the lift arms at a certain height for a given position of the lift handle, and the lift cylinder holds enough pressure to lift the load to the proper height, but then the valve closes so it doesn't lift any higher, but then it leaks off slowly and the arms drop, and then the control handle linkage kicks in and opens the valve and lets more pressurized fluid from the pump go back into the cylinder to lift it back up , and once it gets to the proper height the valve closes and it begins to leak down again...
You'll need to pull the lift cover and rebuild it. A rebuild kit is only about $20.00, and once you have the lift cover off it's not that hard to do. The hardest part is removing the lift cover and getting it back on, as it is heavy. You will need a helper or an overhead hoist or engine crane.
