Mixer shower goes cold during showering the hot again.

You say you have changed the unit assuming this means the shower Is it a thermostat unit and you say boiler pressure is fine What type of boiler if not a combi how is the water heated With these details perhaps you can be helped further Would suggest you remove shower unit and check any filters in the pipe work have you checked that boiler is still fired up when it goes colder as if you have restricted flow the boiler probably is shutting down

Have resolved this issue several times on Baxi & Ideal Logics The problem is the flow turbine. If you changed the shower unit then you know it’s not the issue, also no other taps are having the problem. That only leaves the boiler You’ll need a heating engineer to check the flow turbine sensor light. First run the taps....(with boiler cover off) and carefully look at the flow turbine sensor light and it should be a solid red. Then turn off your taps and turn on the shower and see if the flow turbine sensor light starts to flicker very very slightly, NOTE: With shower on, the flow turbine sensor light will only flicker very very Very slightly that you can only notice when you look at it very carefully but you should notice the difference when running taps (which is a solid red light. Taps on: Solid red light Shower on: Very slight flicker If you see this. simply change the flow turbine (Not the sensor) and your problem should be solved. Your sensor is working that’s why your water is going cold. It’s the magnets in the sensor that have limescale built up. I have fixed this problem a number of times but I have no idea why the problem only occurs with the shower on. Hope it helps

It could be the cartridge in the mixer unit which is causing it to go cold

Try turning a hot tap on slightly whilst having a shower and see if problem persists, if this cures it there could be a problem with turbine sensor on boiler.