David,
Sounds mechanical - the first place to start lube the under side of the joystick at the acutator, then if that is not successful likely it's a sticking valve inside the body of the control lever. OR if you only expereince this after remounting the loader, could be air in the system., but based on your description I doubt that.
Also, if this happens under low rpms, and not higher rpm it points to the valve/seal area or could be a combination of Mech/Hyd in this case. e.g. higher pressure causes the valve to return to position.
As for a fix, this could be any one of three valves, so have a o-ring repair kit handy if you remove. I have done this before with other equipment (r/r loader valves) and you if you don't need to do this then don't.
Carl