How to fix my Worcester Bosch 24 CDi Combi boiler?

My hot water causes problems in the shower, which has a mixer tap. The temperature is fine, then goes cold, then goes hot again in a perpetual cycle, about every 3 minutes. The Central heating is always fine. I suspected water pressure problems, or heat exchanger issues, but have had a service and was told there are no problems with either. Strangely enough I bought a new shower head, which sorted the issue for a few months. PLEASE help?