When running hot water at less than full on (shower typically) boiler will go out following 'kettling' then relight.
Flame does attempt to modulate ok, reducing flame as temp rises, but probably not enoguh for it then goes out only to relight on cooling.
Both diaphrams on the changeover unit...
On a TT circuit there is a split load CU where the main incoming RCD trips for no reason, even when house empty, even overnight; its a Wylex WRMT 100A 100MA of uncertain age! Suspected earting loop initially - can remain ok for weeks on end, then trip off 4 times in an hour!