So, I wonder if this had anything to do with the irratic cooling?
Still doesn't explain the drop in coolant level, but all the lines and hoses were checked, system pressure-tested, fans and thermostat tested, and a leak-down test performed on the engine. It was then test driven for a week.
Everything checks out fine. It's so weird!
Could have been an air pocket, wet sensors, or some other odd thing I guess.