boiler problem - hot water works then cuts out after a few seconds

hi all we have a vokera compact combi boiler and the hot water isnt working properly.... basically, if you run it really slowly, then the boiler fires up, and then you can get hot water and turn up the flow if you just turn the tap on as normal, the light goes red on the boiler and it has to be switched off/on again any ideas anybody? thanks
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