Coolant disappearing
 

New radiator, new heater core, fill reservoir has no leaks, hoses are good. Gauge never goes over halfway, except when the overheat occurs at that point it's still halfway until you fill the reservoir then it goes to hot if you start the engine the fans come on and the temp comes down,drive for a couple of hours and it starts again.. There's no water in the oil, no steam out the tailpipe, I put 16 lbs on on the system holds for over an hour. Thought it might be the cap except there is nothing in the overflow side of the catch tank. Could it be a stuck thermostat, that would restrict flow and cause an overheat which should pop the cap and overflow into the catch tank, but it's always empty. I did fill it with water and am waiting to see if it has a leak. Any ideas.

get a pressure test see if anything is leaking

I have exactly the same problem and was told it was a leaking head gasket. Not blown but leaking somewhere.