engine cooling system on 1997 Chevrolet Express 1500

i have a 1997 chevy express van 5.7L 1500., with a cooling problem. the overflow fills up after 20 minutes or idling for 10. the thermostat has been checked and opens, the radiator cap replaced, and only leaks after boiling point out of the overflow hose for that purpose,....
I need to know what you think the problem is. its got 164K miles, everything else works great,...airconditioner is also a factor of the overheating radiator. there are no lights coming on from my dash warning lights,...if you have a question for me please reply to

The problem is most likely a head gasket. There are water ports near the cylinder head gasket and it could have a small burned area that allows some engine compression to enter your water stream. This in turn will build up pressure in your coolant collector tank. Best way to check is to have the water tested for carbon monoxide. Call around to some radiator shops and ask if they can test it. A good shop will have the equipment, composed of a stopper and hose that fits on the tank and goes to an electronic sniffer. Good Luck