What's wrong with my Worcester 28CDi RSF Boiler?

My boiler is approx. 6 years old and covered by BG Homecare, BUT....... after fitting a thermistor sensor, a multi function gas valve and an air pressure switch they have not resolved the problem of constantly fluctuating hot water temperatures from all my taps and 2 Mira showers. BG engineers blamed the showers, but the Mira engineer has replaced the cartridges (though he said they were not faulty) and the problem persists. Regardless of what setting the DHW control is set to, the hot water comes out scalding hot one minute and cool the next. PLEASE HELP !!!!!