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Hope this helps!
Do you have Home/Away Assist turned on? Many users have problems with that feature. I just leave it off and control the Nest with a schedule or manually. Have you tried that?
(I can't tell from your post whether the 80 at night is due to the heat running constantly, or the AC failing to run at all. If someone more expert is trying to help you, more than just what I wrote, they'll need to know whether the Nest is doing too much heating or not enough cooling.)