condensing boiler goes bang?

I have a newly installed, and just commissioned, Ferolli 35C condensing boiler in a new house in France. It worked fine for a week and then I shut it down. It runs underfloor heating and hot water. I restarted it after about 5/6 weeks and it runs for a while reaching operating temperatures. It then builds up pressure inside and goes bang. The casing visibly expands each time. The frequency increases, the' bang ' gets louder,and the temperature drops 3 degrees before recovering- so I shut it off. I am not sure if it is steam trying to escape, or the pilot is going out and the gas is exploding on hitting the hot boiler. Please help. Am I in danger of the whole boiler exploding? The maintainance company ,and the plumber say its " bizarre". Trouble is I will have to run the boiler again for them to see the problem, which is probably not a good idea!!!