Vokera linea ag plus. Domestic hot water cycling hot and cold?

I have the same problem with taps or showers. Checked again and its not just the showers, hot tap turned on and the boiler ignites, flame on for approx 1 and a half minutes, flame shuts down for approx 12 seconds and carries on following this cycle while the hot taps or shower is on. Turned up the thermostat on the heating and that works fine, boiler ignites and runs constant until thermostat is turned back down. Thanks