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During this problem the pressure at the shower head never drops the
water just goes hot & cold. I just tested the hot water tap in the
kitchen where the boiler is situated,so I can view the control panel.
Once hot tap is turned on the green light comes on followed by the
amber light & the water temperature increases to approx 65c this
continues for approx 3-4mins. Then the amber light goes out the green
light remains on & the water temp starts dropping to below 38c the hot
water from the tap starts to go luke warm but pressure seems the same.

Once the water temp drops to approx 38c the amber light engages (turns
on) again & the water temp display shows the water temp increasing
again & the water at the tap starts to get hot again.


You might be drawing hot water at a lower rate than the boilers minimum
modulation output. To check try running a the bathroom hot water tap slowly
simultaneously with the shower. If the cutting in and out goes away then you
have found the answer.
This might be down to a limitation of flow because the shower head has
scaled up or an internal filter is clogging with scale. It could also be
down to water temperature sensor problems in the boiler or a modulation
problem for one of a dozen reasons. Try the first steps above and see how
you go.