In a previous post you brought the X in and they told you it was a wiring problem. Did they elaborate further on the problem such as the AC compressor not receiving 12 volts to engage the clutch or did they say that the compressor was receiving the signal, but the clutch wasn't engaging?

Since the AC system appears to work some of the time and not work all of the time we can assume that there is enough refrigerant in the system. Since you weren't able to provide any meaningful pressure reading (go back to one of your previous post and reply to the comment that I asked about the valves), I can speculate that the reason why you don't have AC is because the compressor clutch is not engaging all of the time. If I had to make a WAG (wild ass guess) I would say that the air gap for the clutch is out of spec. The gap is probably to wide so the electromagnet can't grab the clutch to engage the pulley.

https://www.youtube.com/watch?v=v-nMDFvSjFk

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