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Can you tell me if it improves if you turn the hot water temperature down on the boiler so you have to run the hot water faster?
Is it only on the shower?
The diaphragm in the boiler will have stretched so it takes more water flow through the boiler to fire it.
The problem with a thermostatic shower is that it throttles down the hot water to get to the correct temperature, at the moment that takes it below the minimum flow rate to fire the boiler.
It will get better if you fit a new diaphragm in the valve.
Yes, it will affect the flow rate.
This is why I said turn down the hot water temperature on the boiler to see if it will fire due to a faster hot water flow rate