I would disconnect the battery overnight in order to reset the computer that runs the car. Maybe the timing is off because of a freak electrical surge.
Take out the thermostat and put it in a pot full of water on the stove. As the water gets to boiling, the thermostat should open.
Are you getting hot air from the heater?
Another possibility is that aluminium scale inside the engine water jacket is preventing the coolant from doing it job, i.e., the coolant can't circulate properly inside the engine. Solution: back flush the system.
Another possibility is that the engine was not properly vented and air is trapped inside the engine water jacket.