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I assume there are no codes right?
Check the transmission fluid level. I know that sounds weird, but If this transmission is even slightly over filled it will do exactly what you are describing and won't set any codes. The trick is to get the transmission to show between 86-122 degrees on the scan tool and then check the fluid level idling in park. If a bunch of fluid pours out of the check plug just let is run out until there is just a little dribble then plug it up. It's a terrible way to check a fluid level, but that's all you can do on this model.
The only other thing I have heard about on this type of car is that it will not up-shift until the pedal is released if the exhaust system is restricted. For some reason the exhaust backpressure prevents the TCM from sending the shift command if there is more than 2 PSI of exhaust backpressure.
The only real way to test it is to remove the catalytic converter and drive drive it without it and try to duplicate the problem. You would have to go through the trouble of mocking up a straight pipe, and you are also going to trigger p0720 codes, but other than replacing the cats outright that's the only way I can think of to test it.
Someone might offer an exhaust pressure testing device that screws in in place of the upstream O2 sensor, but I've never used one.
I just wanted to check in and see if you needed any additional information on this question? I haven't yet seen a response from you since my last suggestion and was hoping to help in any way I can.
I hope I have been of some assistance and would really appreciate a positive rating. Please let me know if there is anything I can do to help in the future.
Ok. Well I don't think there is much more insight I can offer on this one without being able to check it out in person and see the problem myself.
I hope you get it sorted out!