Welcome to 2CarPros.
That is an interesting one. I have to be honest, it doesn't sound like an AC issue. The compressor doesn't care if you are going up hill or at an idle.
I have a question. Is there any ticking sounds from the dash when the key is first turned off or on? I'm wondering if there is an issue with what is called a blend air door actuator. It could be bad, have a bad connection, or not be fully attached and only works under the conditions you mentioned. The blend air door actuator is what is responsible for the air temperature entering the vehicle. If not working correctly, you won't get the temp you requested.
Do me a favor, take a look through this link. It is the directions for replacing the actuator, but it does explain common symptoms. See if any of the symptoms mirror what you are experiencing.
https://www.2carpros.com/articles/replace-blend-door-motor
Let me know.
Take care,
Joe
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Saturday, September 7th, 2019 AT 7:20 PM