Ok, plumber says absolutely nothing wrong with the boiler, although he can’t explain why the shower hose jumps around with (assumption) pressure fluctuations.
His recommendation is to set the water temperature to a comfortable showering temperature and not add any cold water at the mixer tap.
TBH if you'd said it was happening at the shower only, Id have looked at the shower but from what you said and it was happening at multiple outlets I'm surprised its not the plate as that's what is common for the description.
Ok, plumber says absolutely nothing wrong with the boiler, although he can’t explain why the shower hose jumps around with (assumption) pressure fluctuations.
His recommendation is to set the water temperature to a comfortable showering temperature and not add any cold water at the mixer tap.