how can I stop my boiler switching off just two minutes into a shower?

My Saunier Duval SD 30e is 10 year old it has started to fault when I run hot water such as a shower. On start of the shower water is nice and hot but two minutes or so later the water goes cold, waiting a couple of minutes without switching the shower off the water becomes nice and hot again, this may happen two or three times during the shower. likewise if I run hot water into the sink for washing up the same fault occurs.The central heating has never faulted, even in our recent very cold winter.