I've had this issue a couple of times before on another make of forklift.

One was a broken return spring for the main lift valve inside the valve block.
Main lift valve was slow to shut, electrically the lift stopped as commanded but if AUX functions where used within a few seconds the forks would creep up at same time due to slightly open main lift valve.

The other was a sticky valve assembly that controlled hydraulic flow between main lift valve block & the turret valve block.

Again aux functions could cause forks to either raise & sometimes drop at random.

It does sound like you have a sticky or faulty valve somewhere in the hydraulic system, I've never worked on one of these so I've no idea where would be best to look.