Car starting problem after heating.

What is your question?

My corolla runs on petrol and CNG (compressed natural gas). I had recently changed my starter and radiator fan (fan is new and starter second hand).I had to change the fan as the car over heated and the fan melted. I have seen that the heat meter shows a point higher then normal on the meter after this. When i do over 10 km and stop the car it doesnt starts again till the engine cools down. The starter works, the battery is fine. This happens more when i drive on CNG. It works for sometime (few days to a week) without problem when i clean the crank sensor. Engine oil is new, radiator water is full, coolent also present. What could be causing this problem.