biasi boiler not working properly?

biasi 24s boiler, I have replaced an airflow switch pipe, and a ch diaphragm, and bled the ch system. the boiler works fine without the cover on hw stays constantly hot but as soon as the covers go on the boiler cuts of the gas for a few seconds and re-ignites then cuts off again and so on the hw temp changes and gets and runs cool. how can i resolve this problem.