High engine temperature

My car's engine is getting hot while driving especially at traffic lights stop. I had send to mechanic for a check and change the thermostats but problem sill persist. Strangely when they took off the thermostat the temperature was back to normal instantly but when they fix it the problem come again. The mechanic did check but there was no leaking from any part of the engine while the radiator fan is working well. I had realized the the radiator water getting dry almost every two days have to top it up. The mechanic has no solution to the problem.