A.O Smith - BFC Hot water Boiler.

I have and operating pressure and gas burner pressure going up and down rapidly, unable to adjust the burner pressure to the correct setting.

I have changed the gas valve but still doing the same.

Pressure at the meter is constant pressure.

Have disconnected the flue, cleared the condensate trap and water in the flue but still the same, check the burner injection all ok.

Any ideals?