Shower Pump has started pulsating any ideas

[Nightsky224]

We had our bathroom refitted 3 years ago. We had a Stuart Turner showermate eco 1.5 bar twin fitted. It has been great, no problems but this morning we turn it on and it seems to pulsate (on off on off in quick bursts) This happens with both the showerhead thats fixed to the wall and with the smaller head on a hose. I tried handing the hose low down to see if gravity would help but still the same.
However I then tried turning the water completely cold and it starts working properly again (as soon as I turn round to hot it starts pulsating)

I wondered if anyone had any ideas?

 

[terryplumb]

assuming hot and cold are from gravity fed cylinder and loft tank ? if so check for air presence .

 

[picasso]

Sticking non return valves.

 

[aelyak]

Check filters