My automatic transmission is not shifting gears correctly. On a hot day and after driving more than 30 miles, it happens that it becomes hard to shift from 3rd gear to 4th gear, or actually, it becomes hard to shift gears after 2000 rpm. It coincides with having a warm day with high temperatures like over 70 degrees and after driving for a long time on a freeway.
I changed the transmission fluid which by the way, it was never changed in probably 13 years. The fluid came out totally burnt. The change in the transmission fluid didn't solve the problem.
What could be the cause?
Thank you.

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2006 Jeep Cherokee Liberty 2.8 CRD