Kettling ideal classicFF

[Twochutes]

My boiler keeps kettling even though stat is right down, 2 plumbers have both flushed and chemically cleaned system. Could it be the pcb

 

[Gasmk1]

ideal classic is cast iron heat exchanger so could just be weak spots in that you may not be able to remedy, whereabouts are you could it be limescale build up if so try a boiler silencer eg sentinel

 

[Peterdevon]

Must be lack of flow through the boiler