Good afternoon my name is ***** ***** I will assist with your query today.
How old is the shower, and have you had it serviced?
Sorry for the delay John, the fault is normally caused by a stuck flow control valve.
When you change the temperature you put more water through the element which increases pressure on that part of the circuit.
The flow regulator can sometimes get stuck causing a shuddering, and typically starts and clears it self just as quickly.
I meant the fault occurs when using the shower and then often while using the shower (without turning down the temp or putting it on eco) the fault \ noise clears.
With regard to fixing the issue I would look to get a plumber out to inspect and potentially replace the flow control valve, and also clean up the pressure switch (as the diaphragm in the switch may also be responsible - though unlikely)
If you provide me with your postcode I can recommend an plumber in your area.
No problem, appreciate if you rate the service received today.