Alpha boiler problem. Please help

0512win

Hi all
I have an Alpha CD32C combi boiler and for the last 8 months it keeps running cold whilst running a shower. It happens every time the shower is used. I have had the council out to it numerous times and they have replaced various valves/parts but it is still doing it. I have lived here for 51/2 yrs and had no problem with it until this year. I have also replaced the full shower mixer valve (thinking maybe the thermostat had gone) but it is still doing the same with the new shower. Please help

macman

Divertor valve would be the first thing to check.
What specific parts have they already changed?