If the temperature drops at highway speed, it's probably not the thermostat.  You can check your radiator fans by turning on the A/C which should cause them to come on. I had a similar overheating problem and it was caused by air lock.  When you change the coolant in the malibu it is extremely important that the air bleeder screw is open when you refill the radiator.  The screw is at the top front of the engine on the small hose which runs from the thermostat housing to the coolant reservoir.  Don't try just running the engine with the screw open because it doesn't work.  You must drain and refill the radiator.  Lot's of luck.