If you're having your tranny serviced, tell the mechanic of the problem, If you're having it done by a true mechanic, he/she can check it out, if not, it may be something you'll want to have looked at. I'm no mechanic, and I can think of a couple things it may be...I.E. linkage, or computer probs. My ability to troubleshoot the new cars is very limited, so that's what I would have to do....Mebbe someone else has a less expensive way of diagnosing.
Good luck.