Triton 100 XR?

Hi, My shower is about 3 years old, a couple of months ago it started pulsing now it works when it feels like it, the low pressure light is on when the thermostat is turned to warm but if i switch to cold water only the warning light goes out, i have changed the head and flex both of which had lime scale deposits, when it does work the low pressure light stays on.can someone help please?