Why does my Volkera Option cycle on and off when the mixer shower is on?

Have a volkera option combi boiler. Operates fine with hot taps upstairs and downstairs and with central heating. However when Mira mixer shower is running, it heats for about 5-10 seconds and switches off for about 2 seconds and just repeats this - obvioulsy this means hot and cold water when having a shower. Everyone said it was mixer cartridge in the shower, so have replaced, but makes no difference. Convinced it must be a boiler issue, but not sure what?