Can someone help with a boiler pressure problem?

We have a baxi dou tec combi 28 HE boiler that is less then 2 years old. Last week we had to have the diverter valve replaced. The problem now is when we are running the hot water the temperature on the boiler drops and the pressure gauge goes into the red to number 4. We have bled a radiator twice now and the pressure goes to 2 but as soon as we run hot water the same problem happens.