1430midget

having problems with my 1999 galaxy v6 autobox, when towing my caravan after 40-50 miles the gearbox will not drop down the gears properly. and will not go into 4th gear at all even down hills on overrun. if i set off from traffic lights it stays in 1st all the way up to the red line. when the car is not towing all is okay. just had all the oil changed and gaskets etc. any help would be useful .