overheating problems help please!!!

ok so i have a 1987 honda civic with the d16a1 swap from the 88-89 integra and i have a over heating problem i replaced the thermostat twice one honda one after market. Rad. cap has been replaced. ok so heres the problem after driving on the freeway for it seems right at 50 miles the car starts to overheat so i pull over and crack open the bleeder valve and nothing but steam exit's the valve after adding 1/2 gal. of water / coolant. i can drive for another 50 miles to repeat anyone have any idea's? btw theres no signs of head gasket no water in oil, oil in water, nor steam in the tail pipe.