DHW problem Vaillant Turbomax Plus 828E?

If I open the cold water supply valve to the boiler fully, when drawing hot water the temp. rises to approx 83 degees. The gas valve then closes fully, the water temp. drops quite a lot, the gas valve then reopens, the water again reaches about 83 degrees. This cycling continues. If I reduce the flow rate through the boiler, on opening a hot tap, the water again reaches 83 degrees, the gas valve then modulates (not closing completely), the water temp. stays around 83 degrees. It can`t be anything to do with the heat exchanger as the more water you pass through it, the greater the demand should be for the burner to stay on. If the heat exchanger was not working efficiently it should still call for more heat from the burner, the more water that passes through it. I would be gratefull for any suggestions.