shower goes hot then cold.... repeatedly!?

I have the same issue with the same boiler.
The heating is fine, the hot water from the tap or shower cycles hot and cold. I hear the boiler shut off when it turns cold no error lights then it fires back up with hot water again and then repeats over and over.
I have replaced the diaphragm, replaced the temperature sensors. Thinking of changing the dhw heater matrix or diverter valve.

Hammer it!
February 2012