1991 Quest tranny problem

My 1994 Quest runs good 178,000 miles. Ihave a problem with when it shifts in town lower speeds, sometimes it shifts hard and hits pretty hard, other times it shifts smooth. When driven on the highway sparaticly it shifts hard but not all the time. When you get up to 60 mph it should read around 2200-2300 rpm. The tranny will sparaticly shift and down and the rpm's jump to 3300-3500 rpm's. I have had it to the Nissan shop twice and they can't get it to do anything. The puzzle is it doesn't shift hard all the time, but its getting more often with the down shift on the highway. Any clues?

I would start with replacing the throttle position sensor.