I'm the OP, just thought I'd update:

My idle RPMs still drop & fluctuate a bit sometimes when I stop, but I found out my AC refrigerant leak is coming from my compressor. A nice mechanic looked at it with yellow glasses and a special light and there was green dye already at the seam of the compressor. He said if it were him, he'd just keep recharging the system whenever it got low instead of paying $800 to have the compressor replaced. I would be fine with that if it were not for the fluctuating idle RPMs which are clearly linked to the AC system.

So? I'm pretty sure I'm going to replace the compressor myself. It's 200 for a new Motorcraft compressor from RockAuto, and then about $100 for vacuum compressor/manifold gauges/PAG oil from Ebay...

ChrisFix has a couple really good videos about replacing AC compressor and pulling vacuum & recharging. Doesn't look all that hard, so I'm thinking I'm going to do it myself, but just haven't pulled the trigger on a new compressor yet.

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