Your boiler is struggling to detect consistent water circulation because trapped air is interfering with its internal sensors.

DIY fix possible

Check the pressure gauge on the front of the boiler to ensure it is reading between 1.0 and 1.5 bar.Bleed all radiators in the house starting from the ground floor and moving upwards to release trapped air.Top up the system pressure to 1.5 bar using the filling loop if the pressure dropped while bleeding radiators.