Baxi solo wall mounted boiler problems?

My boiler fires up and appears to stop when the boiler reaches the thermostat level. It doesn't fire up until after a good while even though the room thermostat is calling for heat. I don't think the pump is on when the boiler stops firing?
I've had room and boiler thermostat replaced, three way coupling and timer clock have also been replaced. It is a baxi solo WM 50/4 RS