Glad people liked that comparison. I came up with it Monday on the side of the highway in the rain while waiting for the jeep to cool enough to figure out what the issue is.
So here's the deal, filled with coolant, everything's awesome. I drive about 50 miles... overheats! I stop, overflow tank is full, and its blowing air into it. Once it cools, it sucks the tank dry. Obviously I lost coolant somewhere, either causing the overheating, or once it overheated, it blew the coolant out the tank overflow.
My mechanic is my buddy, so we spent all day yesterday going over it... for the second time. The radiator was changed by a previous owner, and looks pretty newish. Last week I changed the thermostat (because it was overheating) on the side of the highway (in the rain... only breaks in the rain on the highway). We changed the waterpump last week also, which was newer but was weak. Flushed the system, backflushed the core, and changed hoses. Worked awesome till Monday, when I overheated.
We can't find a coolant leak anywhere, and there's no white smoke. Yesterday we went over every hose, every connection, every clamp (changed most to hose clamps, rather than spring clamps). We drained and flushed the coolant, pressure tested, put in Peak flush solution, and dye. I drove around for 5 hours on the highway, and in stop-go traffic... and no overheating. Everything checked out fine! One theory was combustion gasses leaking into coolant, and causing the overheat... but pressure tested perfect.
So, it seems it has fixed itself. My current theory is either a clog somewhere which finally opened, or air bubbles (even though we made alot of effort to make sure we didn't have any major air bubbles in the system last time). I will be watching the coolant closely, to see what happens. If it was a bubble, it was about a gallon sized bubble!