95 Tranny Problem

I have a problem with my 95 Millenia transmission. Every once and a while the car will not shift out of park to drive, or, while driving, the transmission will occasionally slip out of gear into neutral. It will re-engage after shifting around through the gears for a few seconds. The car has an automatic transmission. Any ideas would be appreciated.

I am having similar problems as "miliman". Our car was sluggish when changing gears. Took to have oil changed and trans fluid changed. On the way back home, while goiing up a small incline, the car went into reverse and started going backwards. I pulled into a driveway and now I can't change gears at all. They seem to be locked up. What can I do? Help.......