Myson Apollo Boiler - Intermittant Fault?

I have had a problem for some time now that I have had people out to look at and many parts replaced within the boiler (Myson Apollo 50i). What happens is that the bolier fires up all OK and after some time (it varies from 20 mins or more) the main burner shuts off but the pilot remains a light, after a very short while, can be imediate, or 10 mins the main boiler will relight and the on/off cycle continues.

What is different is that I have found out that if I remove the front boiler casing (exposing the burner etc.) the boiler works faultlessly, however when I replace the outer casing the problem starts again.

As the boiler works OK without casing cover I am reluctant to replace the boiler as recommended by last engineer. Circuit board, hi / low stats, etc all replaced, new circulation pump also fitted