1999 Transport trans problem

Hi group,
I have a 1999 Pontiac Transport Montana. Normally I only drive this car fairly short distances (under 25 miles), however recently I've had to do
some longer commutes, and have noticed a problem.
After the van has been driven about an hour at highway speeds and then into city drivng, the transmission will shift VERY "sharply" between gears. This never happens if the van is driven shorter distances, only if it's been run on the highway for an hour of more. Once the van cools down the problem goes away.
I tried having a additional trans cooler installed, and this hasn't had made any difference. Is this a sign the trans is "done", or is it a fluid or some other issue?
Again, it's a 99 pontiac trans sport, 3.4l engine, and has about 110,000 miles on it. The trans was replaced under factory warranty at about 30,000km due to a defect, so the trans has roughly 80k on it.
Thanks for any insight!
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I have the exact same vehicule and experienced the exact same problem. I replaced the pressure control solenoid and that fixed the problem for about 15000K miles. Now it's back again. :-(
Apparently dirt buildup inside the trans may cause PCS/TCC solenoids to work incorrectly. I would start by changing the trans oil and filter and see how it goes.

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