Hydraulic blade control.

problem solved

Any chance the oil is foaming? If not, I would look at the control valve.

Usually there are springs that center the spools as soon as you release the lever. Either the lever and linkage may be binding and delaying the spool centering or maybe one of the springs is broken. They are usually contained inside the valve block to keep trash from accumulating, usually with a cap on each end of the spool.