i would check the lowering circuit first
try disconnecting the wire from the lowering dump valve solenoid and see if it stops. If you get results there then back trace the circuit till you find the shorted component. it also could be a faulty valve not seating enough and allowing it to drift down.

Then if you do not find any electrical problem with the lowering circuit then i would move on to a mechanical issue with valves such as check valves.