PT Cruiser over heating?

2001 PT Cruiser LE 2.4L, red line over heats when driven. Will boil out the coolant threw the overflow tank while driving down the road. Running the heater reduces the heat slightly. Fan works properly. I removed the thermastat in hopes to keep the temperature down, which did not help. The radiator does not get hot, but it does get lightly warm. Could this be a clogged radiator? the radiator does not show any external damage, and there is no leakage.