2000 Pontiac Montana . Goes into gear hard when it shifts. Has 140000 miles on it.

It sounds like the pressure control solenoid inside the transmission. With that many miles I would suggest going ahead and rebuilding it. You need to remove the transmission from the van anyway to replace the solenoid. I have rebuild several of these, be sure to put in a shift kit, it makes a big difference. I've rebuilt them before and wasn't happy with the way it shifted, so I pull it back out, put in the kit and that did the trick. So, needless to say, I always put the kit in the first time. Although, I do know a guy that changed his filter and added some Lucus oil treatment for the transmission to the new oil, and he says he hasn't had that problem sence........makes me wonder.

When I found that the hard shifting stopped after I let it cool down on the side of the road I started looking into it. Seems the PCM or power control module was involved. An OBD reader was able to delete the code and this worked for a while but seems the real problem is with the front O2 sensor. Any number of things can make it mess with the tranny and cause hard shifting. Check the code P1133 and you will see what I mean.