Yes it does sound like a blown head gasket. It passes the pressurizing test, but leaks when combustion pressures act on it. Leak path is combustion gases into the coolant, causing an overpressure condition. When you get around to it, try looking in the radiator when the car is idling and look for bubbling or foaming which would indicate a blown gasket. Better yet, radiator shops have a way of detecting combustion gases in the coolant. Sinc your problem appears subtle, you may want to pay them a visit.
95 E320 Cabriolet, 131K