Worcester Highflow 400 RSF CH or HW but not both?

Boiler about 9 years old serviced regularly.
Boiler basically operating OK (programmer, ignition etc.)
Can get HW OR CH separately OK.
When CW on cannot get HW.
Switch off CH and HW OK.
Sometimes CH overheats, trips out and loses pressure.
Pressure runs at around 2.4 bar.
Whats up please?