Reply to "What the heck just happened?"

I had a similar problem with leakage at the overflow tank cap. After replacing the pressure cap, it still blew steam when hot. I examined the flange where the rubber cap seal is pressed down to seal the tank and discovered that it was scarred and uneven, resulting in an improper seal that would blow by when the pressure was high enough.
I found a large 1/2" drive socket that fit down into the cap "cup" on the tank. I glued sandpaper to the flat side of the socket (probably 180 grit), then I pressed down and turned it back and forth several hundered times until it was clear that I had a continuous, flat seal face. My tank was brass, so once I got through the chrome plating, it cut nice and even until flat.
It is just another detail to check. The system has sealed up nicely ever since.
Look in the past postings for detailed info on properly bleeding air out of the cooling system. This is the most important step to getting a cool running system.
Also, if you decide to replace the thermostat, check the past postings to make sure you get the right one. Do NOT trust an auto parts store to give you the right one.