Hello I'm the new member here.. I have 1979 Trans Am 400 6.6.. I could not find the problem of overheating when drive over 55 mph.. I changed thermosat new.. Drain n flush the antifreezes.. The hoses are in good shape. It's rebult only maybe 1,000 miles on it.. The fan is on always.. No clutch.. It has 4 core radiator.. The cap is new.. I did see the water move around when I open the cap and my friend gas the motor..I need help to solve the troubleshooting... I am waiting for your reply.. Thanks!!!!!