Valliant Turbomax plus hot water problems?

Have a Vaillant turbomax plus boiler, which has been working fine until a couple of months ago. When turning on the hot water tap, once heated the temperature was constant, now it is not constant, goes from hot to cold to warm etc. Sometimes the boiler will not fire up to actually heat the water, so turn tap off and try a few times until it does - but still not constant temperature. Any ideas?