Reply to "What the heck just happened?"

My oil is clean. I looked at my cap - it is a 16lb Motorcraft cap. I sanded the top of the tank with an orbital sander - then put on a 13lb cap which fit snugly. No bubbling/smoking or spurting of coolant. Engine purrs like a kitten. Kind of a boring conclusion. Thanks everyone for your help though - part of the fun is learning about the car and how it works. Because of all of your suggestions I was able to think through the problem and come up with the simple fix rather than throwing money at the problem or throwing my hands up in frustration. I suppose I could still have a blown gasket, time will tell - I haven't checked the compression yet. Thanks again.