Airmatic question, sometimes sits low in the rear when I turn off the engine and sometimes it looks normal for days?

From the description it sounds like one of the dump valves isn't seating properly every time. The way the system operates is by determining a level car, you get in or put weight in the trunk, it determines the rear end is low and adds air. You drive to wherever and remove the weight, the car then determines it is too high and opens a solenoid to release air until it's level. In your case, that valve sometimes doesn't seat fully, and it leaks. The biggest reason this happens is because of moisture in the air. Whenever you compress the air its get hot. The air also contains moisture. That warm moist air gets pushed through the system. It contacts all the areas that have metals, and they start to corrode. Then you get leaks if the corrosion is in the valving or fittings. Your car does have an air drier on it, but I suspect it's the OEM unit that hasn't been operating and it's letting wet air into the system. As such what you really need to do would be to pull a vacuum on the system to pull moisture out. Then change out the air drier and the solenoid that controls the air. These are all mounted near the compressor at the front of the car. You can get a replacement Arnott solenoid unit for around $200 and a replacement compressor drier for around $300 through Rock Auto. To install it you need to disconnect the battery, then drain the air out of the system. Now what I usually do if I'm working on an air ride car is to go under it before I do anything and set blocks under the suspension and frame at the current ride height. Then I jack it up, do the work on it and once done I set it back onto those blocks. Then reconnect the battery. That way it shouldn't need to be recalibrated for ride height when you're done.
There is also a conversion kit, but as you state yours does hold that sort of rules out the lines and air springs and points to the valves. If I was in your place, I'd probably just replace the valve for now. It will likely last for as long as the rest of the car at this point.
Oh, and the part about opening the door or trunk causing it to lower, that is normally how they work, it's supposed to make it easier to load the trunk and ease your entry into the car. Personally, I'm not a fan of that as I dislike crawling into a car.

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