Car is overheating normally when I first drive.

Car is overheating normally when I first drive to work. Temperature gauge goes up and then comes down rapidly. I have not been able to find coolant leak but car is losing coolant. Radiator is new, engine gaskets, timing belt, water pump were recently replaced. How do I diagnose problem my mechanic has not been able to find problem. Also noticed that when car over heats I am not getting any heat in car.