bolier comes on then goes off

Hi i live in the uk I have a vaillant vcw gb 282eh combi boiler can anyone tell me why when i turn on my hot water tap the boiler fires up and i get hot water. about 1min the boiler goes off then fires up again this happens has long as the hot water tap is running. no blockage in both heat exchangers the all sensors/safety switches work but no why of telling please help