Bob DeCamp, this is going to be tough on. This could be as simple as
- Low transmission fluid level
- Dirty transmission fluid
- Shift solenoids
after that you could be running into major issues. It could be a valve body problem as well as the front (OD) servo or a bad OD band (unlikely since your trouble start after some driving). I'd start with simple (inexpensive) to complicated (expensive) as well as a good manual