I have a baxi bermuda inset 2 backboiler that cuts out when you turn the roomstat up for the radiators to come on.We had the same problem about 9 months ago,so called an engineer who serviced it and couldnt find what the problem was,but when he had finished the service it was working fine again.He said it could have possibly been some dirt etc on a sensor.When the roomstat is off the boiler works fine just for the hot water,but as i say if you turn the roomstat up it cuts out after 60 seconds or so.