Chrysler Town and Country Doesn't Want to Shift to Overdrive

My parents recently bought a 97 Town and Country automatic with 3.8L
V6. The previous owner had the ATX fluid and filter changed, and it
shifts very smooth. Most of the time it shifts through all gears,
including overdrive, but sometimes it won't enter overdrive. There is
no rough engagement when it works, and when it doesn't work it is
almost as if the gear selector was left in 3rd position - it shifts to
3rd and won't go any higher. The only way to correct it seems to be
to stop the vehicle, put in park, and restart it. After that it works
fine. Any idea what the problem could be? Check engine light is not
on.

Thanks.