99 Montana shifts hard.

When the transmission warms up, it starts to shift hard, not really clunky but jerks the car forward through all the gears during acceleration. Fluid level is good, and doesn't do it in on short trips, but after a few miles on the highway, if I stop then go again it shifts rough. Any ideas? I have had this problem since I bought it 4 years ago, and it hasn't gotten any worse, is it something to be concerned about?

When the transmission warms up, it starts to shift hard, not really clunky but jerks the car forward through all the gears during acceleration. Fluid level is good, and doesn't do it in on short trips, but after a few miles on the highway, if I stop then go again it shifts rough. Any ideas? I have had this problem since I bought it 4 years ago, and it hasn't gotten any worse, is it something to be concerned about?

Stucked or seized PCS.
Presssure doenst go down when transmission shift.
There's 4 Pressure Control Solenoid : 1-2, 2-3, 3-4 and a master one on 4t65 – e, enhanced transmission, auto 4 spd, hmd (3,4 litres).

I fix the problem by following instruction :
- drain the trans,
- change oil, filter and gasket,
- add Lubegard Automatic Transmission Fluid (ATF) Protectant http://www.lubegard.com/C-112/LUBEGARD+Automatic+Transmission+Fluid+Protectant

... and it unstucks the PCS.

BTW, I resell the PCS that I bought for nothing : http://www.leboncoin.fr/equipement_auto/162641296.htm?ca=13_s :-D

Rgds.