I'm Neal and I am here to assist you. Only accept when you are happy with my assistance. If I can't help I will open your Question to other experts.
I would suspect based on my experience that you have a very small head gasket leak, and you do not want to replace the head gasket as it would cost more than the car is worth. I would try a product called Blue Devil, Ihave had tremendous success in cases even worse than yours. It is a high endblock and head gasket sealer. Most parts stores carry it. Make sure you follow the instructions and chances are extremely good it will fix your issue permanently or at least for a very long time. They advertise a 85% success rate on the northstar engines. Here is a link to the manufacturer if you want to read more about it.
I cannot say for sure about the stalling, that could be fuel pressure. Coolant leaks can occur other places but you said that a pressure test showed no leaks.
I have seen a lot of these and it is usually the head gaskets.