i have had this problem for some time i describe it as 'revving up' The water does go cold for a few seconds in the shower - i have had a thermostat replaced twice and am now told the problem is the temperature difference between the in and out flow and that to repair this i need to flush out my complete system - Some radiators are cold at the bottom ! - is this likely to help or is it some other fault ?
I've had this very same problem with this boiler, twice! Both times, the repair men replaced the circuit board, the pump and a couple of other things. Eventually the first guy changed a small rubber hose about 3 inches long up at the top right of the boiler. It was blocked. Once replaced all was good. The second time, the repair man said it could never be that so he replaced almost everything. Eventually he tried the tube and yep, it was blocked again. I now change that tube every service.
Doesn't help with my banging noise inside the boiler though!