Here's something else. Take the air cleaner out of the car and check the thermostat. It fails in the hot air position. The car continues to draw heated air from around the exhaust manifold even after the car warms up. A simple test would be to remove the hot air duct from the air cleaner and see if the car runs better when it's hot. The duct is the shiny hose that runs from the bottom of the air cleaner, just below and aft of the radiator, to the exhaust manifold.