Gloworm Ultracom 38CXI Hot water issue

[JAY123]

Our combi boiler is having a very random issue.

When the boiler is on the DHW and CH mode the thermostatic shower and Bath mixer tap both drop in temperature after about 2 minutes of running, but when the boiler is on just the DHW mode the shower and the taps run totally normally.

We have previously changed the heat exchanger and the diverter valve in the last 2 years as well as the PCB board about 4 years ago.

Any ideas of what it could be would be much appreciated?

Thanks