Clio 1.2 P registration engine dying when it's hot?

My Clio 1.2, P registration runs perfectly well when the weather is cool but gives me trouble when it's hot. It only happens in hot weather and when I have driven over 100 miles. When I have to stop at a traffic light or roundabout it suddenly won't pull away. The engine will start and idle but there is no power. It is as if it is over heating but the temperature gage shows normal. I have to let the car rest and it will go again. I had the RAC out but by the time they came they could not find any thing wrong with it. My garage thought it might be the coil and fitted a new one but it has happened again. Any ideas ?