Still trying to figger out what's happening to my coolant

[Skunksmash]

87 tbi 350. My coolant is still going somewhere, but I have no idea where. Its not in the oil, its not on the ground. But every so often, I have to put a couple quarts in there. Also, you can't fill the overfill jug to the line. If you do, it will all be coughed up out the overfill lid. Weirdest thing I ever saw. Just takes a few days time.

It finally got a little cool here in TX so I decided I needed to turn on the heater. It didn't work, so first thing I did was check the coolant. It was low. Filled it up, heater worked again. So any idea what the heck could be going on here? Mysterious vanishing coolant. And a puking overfill jug. But only if you actually fill it up to the fill line. If you just leave I just leave it dry, I don't have that problem. But I'm wondering if maybe its a symptom.