mondeo tdci auto box problem?

hi i own a 54 mondeo tdci auto when its cold its fine but after 30 miles from 2nd to 3rd it drops into neutral take your foot off the gas and it goes into 3rd if i leave the car for a while say go shopping come out it,s fine again the wife only drives it locally so she has never had the problem its just when you go for a run out some where can you help ? thanks mark...