Define "overheats"- - - -does the coolant boil, or does the temp gauge just move into the "hot" zone without boiling over? Have you actually measured the temperature of the coolant? I use an Infra-red thermometer to check the temperature of various areas of the block, hoses, and radiator, and particularly the temp of the area immediately above and below the thermostat.
It's entirely possible your temp gauge is lying to you. What pressure rating is the radiator cap you're using? A 7 lb. cap will allow the coolant to reach 233 degrees before it boils. Go to a 12 lb. cap and your boiling point goes up to 248 degrees.
Jerry
