some other reasons for car overheats

if may add,  a clogged radiator may also caused the engine to overheat. To determine whether actually it is clogged.

   1.  after you have removed the radiator cap, fill it up with water, do not replaced the cap, leave it open, start the engine, using the accelerator cable in the engine, accelerate the engine, look at the water in the mouth of the radiator if it is not moving  or running  a little bit faster, if so then two possibilities.
               a.  the radiator is clogged because the water pump has difficulty sucking the water .
               b. the water pump is not functioning.
How to check a clogged radiator.  While the radiator is full of water, remove the drain plug of the radiator, if the flow of water coming from that hole is not that strong then most likely your radiator is really clogged.

How to check a defective water pump.

        a. detached the upper radiator hose at the side of the radiator only.
        b. fill the radiator with water.  start the engine, accelerate the RPM of the engine , the coming out of the water at the end of the radiator hose must be strong, if not, it can be assumed that your water pump is not working properly.

THE ABOVE CONDITION MAY HAVE BEEN THE REASON WHY A CAR OVERHEATS DURING IDLE .  this is because at idle, the water pump is not moving fast and therefore the water from the radiator stay stagnant or not moving well toward inside the engine to cool it off. While if you run the car faster , the water pump rotates faster and at the same time the radiator is receiving more air due to engine velocity. To prove this, at idle, run engine in a higher RPM,  if the temperature gauge goes down then the above may have been the reason