sluggish 727..

hi all.

whats the usual culprit on a 727 when it wont allow you to start out in drive without putting it in neutral first and waiting a moment for the converter to fill. front pump maybe? u can put it in drive and go when its hot only. even then when driving though it seems to slip a bit (car not keeping pace with engine revs) especially up a grade. yes i know a rebuild is likely but just looking for a possible cause thats maybe well known and gives those symptoms?