How can I fix my Worcester 28CDi Combi Boiler?

Intermitent problem.
When running the hot tap the boiler and the pressure gauge start to pulsate. A new heat exchanger was fitted nearly a year ago. The boiler is about five years old. Although this is a hard water area, I have an electronic scalemaster fitted. It is working well as I only have a little lime scale on my sink taps & shower head and it is easily removed. There is no problem with the heating side of the boiler.