Sounds like a bad cap. Many places have a pressure fixture that can test a cap but they are pretty cheap so...

You may also want to check the coolant system hot. CAREFULLY remove the cap when the engine is warmed up (and after mysterious losses) and then test. It is possible that some coolant is being lost into the engine via a failing head gasket or crack in the head. In that instance you will likely see bubbles in the radiator when warm and revved with the cap off (or not, they can be hard to detect in an x-flow unit) Pressure testing with the engine running may provide useful info as well (varying pressure as the affected cylinder alternately draws coolant and injects gasses). An oil slick on the top of the fluid is a certain sign of a leak somewhere as is coolant in the oil.

Mike!