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Hi I'm Brian and I'm here to help!
This points to one of a few things but the most common of them being that the run capacitor in the ac unit is weak or has failed. Other possibilities are: a loose or burned wiring connection in the unit or at the breaker(s), a failed starting capacitor or relay (if equipped) or a failing compressor that is seizing up. I would look at and tighten all wiring connections in the unit and the power branch circuit going to the unit. Then I'd replace the compressor run capacitor. Chances are those things will take care of the problem.
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It sounds like it's right on the edge of tripping the breaker or heating up and shutting the compressor down. One of the two. When it cools off and there isn't any solar load and the ambient temperature drops, the amp draw of the compressor and the high side refrigerant pressure both drop. That is why it continues to run at night. Still sounds like a loose wire connection. Sometimes they're not visible, you need to tighten all connections including the breaker connections and connections in the disconnect on the wall by the ac unit. There's also a chance the condenser coil is dirty enough to cause this issue and it only shows up during the heat of the day with a solar load as well. Clean the condenser coil with simple green and a garden hose. Tighten the breaker connections and any wire nutted power connections. Double check that the larger of the two refrigerant lines is beer can cold while the unit is cooling. That should do it!