2000 Chevy Lumina hard shifting

Sometimes this car hard shifts into gear. So far I have been unable to relate it to the way I drive, the heat or the length of time it has been turned off. Many times after it has sat in a parking lot (off) for a few minuts it is just fine. It may be just fine for the rest of the trip or it may start to hard shift after only a few miles I have driven it for weeks without a problem and then the next time I back it out of the driveway it starts to act up.