If you leave the radiator cap at the first detent, so it's loose but not off, start the car up (engine cool). With the engine idling, and the water cool, take the cap off and look at the water level. The water should be still. Keep looking into the radiator, and when the thermostat opens up at 160 degrees, you should see the water moving. The water will pour from the radiator's internal fins like a little waterfall. On the Pontiac crossflow radiators, the water flows crosswise from the driver's side (upper hose) to the passenger side where the radiator cap is. If you see the water flowing when the engine is warm, your water pump is working, and your thermostat is opening.
Put the cap back on, and tighten it down. Pressure should start to build in a minute or two. Make sure the lower radiator hose is not collapsed. The lower radiator hose should have a spring inside to prevent collapse.
If the lower hose is ok, turn the car off. As pressure builds, feel the lower and upper hoses. They should be hard. Again, make sure the lower hose does not collapsed.
And yes, check the gauge. If the engine is hot enough to boil over the radiator, it sounds like you're at least at 230 degrees. At that temp, the engine will run rough (hot fuel), and it should be obvious the engine is hot.
Q: Does the engine run ok, everything seem normal, until you shut off the car off, then the radiator boils over?
Q: The coolant recovery system requires a coolant recovery radiator cap. Is your radiator cap correct?
You may indeed get some boil over the first time you fill the radiator. Water may dump into the coolant overflow bottle, then (as you said) the coolant will be drawn back in after a few hours. On these coolant recovery systems, you don't have to worry about the level in the radiator (other than the initial fill). The overflow bottle will handle the extra coolant, so you only need to add coolant from the coolant overflow bottle.
