1992 BMW 325i overheating

Got a 1992 bmw 325i, it over heats after 30 min running or driving. Radiator coolant leaks from the radiator cap and no where else. So it looses coolant. I doubt it's a blown head gasket because there's no smoke from the tail pipe, no stress on the engine, no power loss, no misfires, car always turns on strong. When I drive it, after 30 min I have to cool it down, take off cap so it can release pressure and cool down, after it cools, it's back to normal temperature and runs good, until it over heats again after 30 min.