Worcester combi boiler cutting out on hot

[Piperdave22]

Replaced a bath tap with shower attachment and Fitted a Aqualisa Midas thermostatic bath /shower mixer . During the shower sometimes the water goes cold for a few seconds and goes warm again and customer said the boiler is cutting out then coming back on when it does this .
Almost like it’s reaching temp ,cutting out then firing back up

What could be causing this and what can I do to remedy it ? It’s a Worcester combi

 

[Aire]

The symptoms are typical of a blocked plate heat exchanger. Was the hot water running fine before you changed the tap?

If the new shower has a greater flow rate than the previous, it may just be highlighting an existing fault

 

[Piperdave22]

Yeah all the others taps seem to be running at a decent pressure .
They were having a similar problem before where they couldn’t get the right mix on the shower and it would drop out the same but all other taps seem ok .

 

[Aire]

Id look at stripping the plate out, and giving it a good clean and see if it improves.

 

[Piperdave22]

I got him to run the basin taps today to make sure it was everywhere doing it and whilst on the phone he said it was running then went cold and got hot again so tells me it’s definately a boiler fault
Only weird thing is he said if the heating is on the doesn’t last as long before getting cold
But if it isn’t on it lasts a bit longer
Have said it’s mostly likely to be a blocked heatplate but could be dhw thermistor or a divertor issue .
Boiler is only 18months old though so I’m hedging my bets towards a scaled up heat plate atm

Any thoughts ??

 

[Aire]

If boilers only 18 months old then get Worcester out to it.