Let me explain a bit about this problem. I work in the machine construction industry, where we fit proximity sensors to machines all the time, which is what the Crank Position Sensor is. The sensors have a very limited range of about 1.5mm. My brothers Clio Mk2 kept failing to start when it was warm and occasionally when cold. We fitted a new sensor which was fine for three months then the problem came back.
I took the sensor off and took it to work to test it. Attached it to an oscilloscope and all was fine, nothing wrong with it. Bearing in mind that knowing these sensors have a very limited range I decided to file out the mounting holes on the sensor bracket in an upward direction so I could position the sensor closer to the flywheel.
We rocked the car back and forth with it in first gear until the
mark on the flywheel was under the sensor.
I placed a 0.5mm feeler gauge between the sensor and the flywheel mark and tightened the bolts.
Since doing this to the old sensor we refitted it and the problem has never come back.
I am convinced that these sensors are mounted at their absolute maximum working distance from the flywheel. Once the engine warms up and expands by the tinyest amount, it moves the sensor to far away from the flywheel to sense it.
This also seemed to have cured the occasional cold starting problem he had aswell.