auto gear box in my transit tourno 3rd to forth problems?

1996 traNSIT TOURNO WITH AUto gear box, It stoped changing into forth gear, so i got a new gear box from fords, now it some times changes and some time not.
and the third gear is taking a lot of revs to change gear. Did i need any other part with the new gear box. One day o.k. some of the time but if i stop for a little while it will only change if i drop down to third by hand
and then change back into drive.