gear box sharan falt?

my gear box in my vw sharan automatic r reg goes into and up to third but not into fourth its like it gets stuck, this always seems to start in the wet. ive tried going up through the gears manuely to try to un stick it but dosnt work. sometimes when i stop the engine and put it in neutrel and re start the car its fine again but other times it can last a few days, this has happened a few times now with wet roads. i saw on other comments it can be to do with the seals in the gear box, but also have heard it can be to do with oil pressure in the gear box is this right?? really want to get this problem sorted but dont want to be ripped off at the garage!