vaillant vc gb 221 h boiler cycling?

My boiler fires up as normal for about 15 seconds then switches off (pilot still on)cuts back in 10-15 mins later and does the same. Tried with just water selected and just heating still the same. replaced safety themostat and thermostat next to pilot flame. pump runnig dont know about the flow rate.both water and heating diverter valves replaced. Mrs say new boiler but im not going to let it beat me.help dont let the mrs be right !!!