Chaffetaux Maury Brittony Boiler 80?

When boiler is in heating and hot water mode, with the heating ON, and we go to use the shower, the boiler fires up and the water goes warm. But the boiler does not seem to go to hih fire and switches off. Then starts up again and the goes off etc. etc. This happens with the HW taps while heating is on.

If heating is off and we run the taps or shower it works fine.

Any ideas on what could cause this, cangeover Valve?

Any help would be great, thanks