Low flow rate swirching off boiler .
There is sometimes a hall effect sensor in the output main. WHen you turn on a tap the hall efect switches on the boiler . Your thermostatic valve opens up when switched on to full flow HOT .
THen as the hot water reaches th mixer bar the thermostatic cartrrdge balance the flow by letting in more cold and reducing the hot side.
The combi boiler hall effect sensor now slows right down , so the boiler thinks you have turned down or switched off a tap, and turns off the flames.
The water goes cold, but as it starts to cool your thermostatic mixer bar turns off the cold and speeds up the hot, triggering hall effect sensor and firing up the boiler.
To see if I am correct ( ish ) and you have a shower over bath ,, then have a shower with the hot tap running slowly , somewhere about 4pint wee rate and see if it runs . If you dont have shower over bath then , if you can , switch your hot water temp right down , to keep the flow rate high , and he hall effect ensor spinning .
PS I am not a plumber or heating engineer.