On Friday, 8 March 2019 12:24:34 UTC, AnthonyL wrote:
Turn shower on, burner comes on, led lights (Baxi 105HE) start to
light up to a maximum. Everything fine for a few minutes then the
burner turns off, leds go back, and wife screams. Burner comes back
on - repeat and rinse.

It depends on combination of the boiler and the type of shower mixer. If the shower throttles back the flow of hot the temp rises and eventually the boiler will cut out.

Some thermostatic mixers and some combis play nicely together, others fight.

Owain