That was my first thought when you explained in the first post, that you had an air pocket in the coolant system somewhere. I've had cars here at the car lot that would do the same thing as yours was, just have to bleed it down and put pressure on it and keep filling. The only other thing I could think was if you had a seep at a certain point on the head gasket. I've seen that also where a car would run up hot and come back down b/c it was just seeping past at one particular cylinder. I hope the problem is taken care of, good luck.