2006 Hyundai Sonata transmission?

Parked car then went out later car would not shift out of reverse. Towed to mechanic. He Said tranny was cooked. He Replaced it. Test Drove was fine. Parked had to move it later same thing happened. Pushed it to a spot on lot. Few weeks later he got into it. Drove in lot. Turned the running lights on and it shifted into reverse once again by itself. At this point its been there for 2months. Mechanic at this point is baffled and I am at a loss as this is my only car and I need it fixed so with that said. Please help and thank you in advance for help