Engine overheating on idle and cooling down when driving

Three months ago, the car overheated and engine stalled. I had the radiator changed. Last month the car wouldnt start because the pulley and timing belt assembly became jammed. I had them replaced. Now the car overheats while idling. The mechanic says he need to open the engine to figure out the problem and that I may need to replace the engine. While I drove back the car from the workshop, the outside temperature was about 38C degrees. I drove over 14kms to my home and the car temperature was normal. The overheating occurs only during idle even its its a short wait at a traffic stop. It goes all the way to H and over but cools down to normal as soon as the car starts moving. What could be the problem? I thought I would sell off the car for spares if its going to need engine replacement. But seeing as how the car is cooling down as fast as it heats up, I would like to know if the problem could be solved by some smaller issues that maybe be economical to repair? Thank you.