While you have the thermostat out, put it in a pan of water on the stove to insure it is opening correctly. Use a themometer to check the temps of the radiator coolant with and without the thermostat installed. Are you using a 50/50 mixture? Is the guage pegging to hot side or just higher than what you think it should be. What range thermostat are you using? 160 degree should be sufficient for summer driving.
