It could be a pressure problem, or a electrical problem, best bet is to try some wynns steering stop leak first in the box, this swells seals and has cured many a auto box for me for years, about £5 so worth it, give it 100 miles to fully work, then if still the same I'd just go to auto box place.
I don't know your particular box, but had recently/ish a shifter problem on a auto box in mondeo that would violently jump up and down gears, repair was £35 for a new part bonus!!, so might be lucky.
tabetha