I would second the thermostats. While you have them out probably should go ahead and pull the lower radiator hose and change the coolant. Coolant maintenance is almost as important as oil maintenance in modern engines. While the coolant is out do a flow test with a garden hose stuck in the top and make sure you have good flow. If car antifreeze has been used/added you might have a radiator full of green goo.

JOhn