Originally Posted by TCP64

Have you checked the cat converter and the muffler for any restrictions or blockage? Is there a leak in the exhaust manifold or piping that is redirecting exhaust gas back onto the engine? If the radiator is not new have you had it cleaned by a shop to insure there is absolutely no blockage? Have you inspected the water pump to insure all the internal splines are still there? Is the fan running at full speed under the hottest conditions? Are the spark plugs the correct ones for that engine? Is the air intake system providing enough combustion air to the engine?