this is somthing i have seen several times, you must restrict the flow at the thermostat housing. i just gut a thermostat put it in.
NOW!! my real question is what pound radiator cap are you running? you need at least 18lbs. i run 22-24 lbs on my radiator. what happens is if you don't run a large enough cap the rad fluid evaporates and/or lets the fluid pass the cap and fill your overflow catch can.
if this does not work you might want to take the heads off and see if you have a blown headgasket.