I've got a newish ideal combi boiler that blows out if it works too hard. You can hear the it ramping up then fluttering and cutting out. I think it might be the flue as I've come across the same thing before and that was the issue. The flue has a little cap on the bend that can be removed. The chamber inside is full of steam which seems odd as Im sure the external part of the flue should be air intake. My theory is combustion products are getting mixed with air intake some how. Any thoughts?