Vaillant Turbomax 242E?

I have a Vaillant Turbomax 242E which seems to be working fine. The problem kicks in when I want to take a shower. I cannot get a decent temperature to shower in. The water is either too hot or too cold. If I turn the cold water full blast and then turn the hot water slightly the water is just too hot. It seems as if there is not enough cold water reaching the tap to reduce the temperature of the hot water. Any help would be much appreciated.