I've got an Ariston Microgenus 27 combi boiler. Recently it's developed
the annoying habit of happily producing domestic hot water for a couple
of minutes, but then shutting down for perhaps five seconds before
starting up again. This means that you get periodic slugs of cold
water, which is especially distressing when showering.

The boiler's thermostat for hot water is on max and just by feeling the
temerature of the flow pipe for DHW coming out of the boiler it does
not seem that it's cutting out on the thermostat, so what else could it
be? It's due its service soon, but I'd like to have some clue as to
what could be going wrong before letting the engineer loose on it.