Ford Taurus 1998

About every 400 to 800 miles after traveling without any difficulty
and coming to a normal stop the transmission goes into neutral and
will not re-engage. I place the transmission into neutral, turn the
key off and then restart the engine, place the transmission into
'D'rive and the transmission behaves properly for another 400 to 800
miles.

Just a guess...

There are so many things it could be, but it's likely you have a shift
solenoid that is overheating after a period of time and causing the
transmission to disengage. By shutting the key off, you are allowing
the solenoid to cool.
Recommendation -- further diagnosis at a tranny shop.

Good luck, 
revbrenda1st

Search strategy:
My husband. (mechanic who apprenticed with Ford, Canada.)