Boiler

I have a fan flued condensing boiler. When it's running I get what can only be described as a minor explosion in the boiler. This immediately causes a sensor to cut the gas supply and the boiler goes out. The fan continues to run and 10 seconds or so and the boiler relights. It then runs for approx 5 minutes and goes bang again. I suspected the fan (my previous posting on fridge fan versus expensive boiler fan). I took the fan out and it runs freely. Could be the motor is getting tired and not spinning the fan fast enough. My thoughts along this line are the fan not pulling the gasses out quick enough. The flame is the correct colour a nice deep blue with short cones just above the burner. The explosion as far as I can see takes place near the top of the boiler. What happens in the area I can see through the sight glass, is the flame goes out. There is no yellow flame shouting down from above or anything like that. Can any one think of another reason before I change the expensive fan. A plumber came and had a look and of course whilst he was here the thing wouldn't bang. He said try a shield as the wind may be playing up the flow of gasses. But it now does it even when there is not a breath of wind. Sorry for the long posting but I am trying to give as much info as poss. Did you get that picture I posted.