tranny will not engage sometimes

Sometimes when sitting in drive the tranny will slip out of gear???
When I push the gas it will go in again around 3k.
This is an intermitent problem and doesn't seem worse in hot or cold weather.The van has 200k km on it and runs great otherwise:)
Can someone tel me where the window washer fluid container is in the back???
Tia
Mike

Transmisson could be dying you might want to start seeing a tranny shop.