1996 Mercury Sable

Hello my mercury sable recently started having a problem. It started a few weeks ago first you had to shift it into N before coming to a stop and then shift it back in D when you were ready to go or else the car would act like you were in N but only if you stopped no problem other than that. Well then I went to go somewhere and when I shifted into D it acted as if was only in N but I could shift it into to R. So after that I let it sit for about a week and my dad came over to pick it up we were putting it on the trailer and it shifted and worked in D. He got it home and drove it around and then he had to do the 1st thing I mentioned shift it into N before coming to a stop and now the O/D light is flashing and won't turn off when he's pressing the O/D button. In the last few weeks I have checked the fluid it was fine normal color and smell. We are completely lost of what could be wrong.