Mf 5455 lift

Where’s the position of the switch when it does happen, in up or down mode or in the centre?
Check the position of the draft/position potentiometer, the top one on the panel on the right side console, check it’s fully clockwise into position control mode, otherwise it thinks it’s in draft mode and will later the hitch height based on the load on the lower arms.
When the rocker switch is in lower position, it will lower the lift as far as the setpoint set by the rotary wheel just behind the switch. When in raise position, the max height controlled by the height limit potentiometer over on the right console (think it’s the bottom one of the 3). If your switch is in either of these modes when it happens, check the potentiometer controls are not being inadvertently moved by accident. If not, one of them could have a dead spot in it causing a bad reading and commanding unwanted action from the lift.
Turn off the transport control also (make sure there’s no red led lit up next to the soft pad yellow button next to the middle potentiometer on the right console).

If none of these checks show anything, could have a solenoid leakage issue on the valve or cylinder seal leakage somewhere.