auto box changin to early?

[Dave-B5]

alrite guys, my v6 satt has pick up some sort of problem with the auto box. it dosnt do it all the time and there nothing wrong when i put it into trip (up & down shift), but every now and there ill be driving along doin 30 in 3rd like it should be but as soon as u go over 30 it changes into 4th! witch isnt a big problem as there enough tourqe to drive like that, but its a bit buggin when ur drivin round and it keeps changin up & down between 3rd & 4th as ur goin through town.
any one got any ideas what it is and how serious it mit be?

cheers dave