Hi,
When this happens, does it feel as though the engine and transmission are not disengaging? Has the check engine light turned on or flashed during this problem?
Since it is only happening after driving at highway speeds, I question if it is related to the converter clutch lockup solenoid. If it doesn't release, it would be like trying to stop a vehicle with a manual transmission without pressing the clutch.
Here is what I suggest. First, we need to check for diagnostic trouble codes which may lead us in the right direction. This vehicle has an OBD 1 system which doesn't require a scan tool to retrieve codes. All that is needed is a short jumper wire or even a paper clip. Follow the directions in the following link and let me know if and what codes are found.
https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method
Also, I attached the diagnostics to test the solenoid specific to your vehicle.
Let me know if this helps or if you have other questions.
Take care,
Joe
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Tuesday, September 21st, 2021 AT 8:13 PM