Fault with Boiler?

I currently have the above domestic boiler, which is approximately 4 years old. Recently I have been having problems with the hot water, when the hot water is running, it will loose temperature dramatically and the water will go freezing cold for about a minute, the boiler will indicate a fault code of E34, when the water comes back to required temp, the code goes off, the above normally happens when running the bath or shower.

Would you be able to give an indication to what the fault would be?