Triton Ivory 2 - PRV - Overflow Problem?

Hi - my Triton Ivory 2 Pressure Release Valve keeps activating. Have checked shower head and pipe - all are ok. Without the shower head on, the overflow dribbles slightly, but move the pipe to a normal operating height and the overflow increases.
Turn the shower on with cold water and the PRV comes straight on
Seems like something is restricting the flow in the shower unit causing the valve to come into play. Anyone have similar problems and a cure?