before you change your thermostat, check the coolant level in the radiator with the car cold(never take the radiator cap off when its hot) . make sure the radiator is full, then make sure your have some coolant in the overflow tank. sounds like you have a flow problem caused by low coolant level. also, air in the cooling system can cause this. don't forget to fill the coolant level with a 50/50 solution of coolant and water.