Radiator expansion tank boils over while coolant guage reads 1/2!

Just recently bought this vehicle, had a real bad coolant leak through the radiator. I went ahead and replaced the radiator, coolant hoses, 50/50 coolant and thermostat twice (twice becuase they can be faulty even though new) The vehicle after about 10 minutes begins to boil over out of the expansion tank but my gauge is reading around 1/2. I know the fan does not kick in till about 3/4 on these (it does) but its already boiling over by then. Also I turn on the ac when its around 1/4 & 1/2 (fan does work) but it still continues to boil over in a few minutes. Im not sure where to go from here, the weep hole on the water pump is not leaking and praying it is not a blown head gasket. Oh while running; the coolant hose from the (upper driver side) engine block/radiator is nice and hot but the hose thermostat/radiator (lower passanger side)is not hot at all, even while the reservoir is boiling over. Help!