I think before handing it over to expensive mechanics, you should this out.
Syphon out the water in the coolant tank, and then release all the radiator hoses and other hoses relating to cooling system. This will empty your cooling system. With the hoses disconnected, turn the car on for a few seconds to let the water pump pump out the water inside it. Do this 2 or 3 times, (restarting the car). Then connect all the hoses except the water tank. Remove it, give it a good clean, check the connectors, and re install it. Fill up the tank with brand new engine coolant, and see what happens. If this fails then i recommend you take it to a professional.
I hope this helps.
Andres