I'd suggest you check your transmission fluid level. If that is normal, then
I'd suspect something wrong with valve body and it need to be rebuilt or
swapped out. I would guessimate less than $1000 to repair this if it is the
case.

Where to have it serviced? I highly recommend the dealer in this case as
they do have computer to talk to the transmission computer and perform
diagnostic.

If you go to any tranny shop, they probably would just say you need a
rebuilt tranny and would cost you $3000 or so.