A combi boiler requires a certain flow rate at the tap to stop boiler from overheating and cutting out. So by cutting back your tap to slow , you are not allowing this and the boiler is cutting out. Open your taps more to allow a good flow rate, if water not warm enough then flow rates have to be adjusted by gas boiler engineer. If this doesn't help u have a diverter valve problem.
simeservicesscotland.com, October 2010
