A flush won't necessarily cure that behavior. Usually flushing a transmission doesn't get rid of certain characteristics.

Typically transmissions shift harder when they're warmed up. When you do a flush the fluid is cold, and in cold weather it takes a while to heat up again. Once you get the car running around for an hour or so, it starts to get hot enough to show the signs of hard shifting again. Also, hard shifting doesn't always mean the trans is on it's way out. Again, just driving around in cooler weather won't ever heat the fluid as much as driving on hills and in city traffic in summer, where the hardest shifting will usually take place.