Worcester 28i Junior Boiler?

My boiler keeps switching off the CH and the reset button keeps flashing once per second. When I switch the heating off, and leaving for half an hour or so, it switches on ok, but will go off again after 15 minutes or so. The hot water is fine as long as the CH is not on. I'm assuming from the manual this is overheating causing the problem, can anyone tell me why? and if there is something I can do to prevent this.