Superior Transmissions on Sligo Ave. in downtown Silver Spring. I have found them to be highly competent, reasonably priced, and completely honest (“That little hard shift is caused by a failing framistat. Live with the minor annoyance. You don’t want to pay to have it fixed.”). Sometimes they are busy, so it’s best to call before you go over 301-587-0512.
(They are in the Mechanics Files on this site. I put them there.)