1996 Sentra automatic transmission problem?

I have had this strange problem recently. When I drive for a while accelarating rather hard at the traffic lights or passing somebody, I start feeling a slight free-go on my gas-pedal, and automatc transmission starts shifting late with some strong "kick-back", sometimes not going to the higher gear at all. After you stop, and wait for an hour or so, everything goes back to normal again. I noticed that accelarating fast is requiered for the problem to occur. What could that be? I changed the ATF one year ago, and generally maintained the car quite well. It has 95K now. Thanks.