If your water keeps pulsing fast and slow or if your pump keeps starting and stopping then you may need to remove the Non Return Valves [NRV's] from the shower.
TO REMOVE THE NON RETURN VALUES (NRV's):-
First turn off the water supply to the shower.
Remove the 'C' clip and remove the NRV.
Re-attach the hose and repeat on the other side of the divertor.
Turn the water back on the shower and check that the pump now works without pulsing.
this does NOT cure the problems, then you may need to check that the
pump is getting enough water supply from your water tank.
If there is water being supplied to the pump, then ensure the pump has been PRIMED correctly (this removes airlocks)
Check the hoses to and from the pump are fitted fully. If air is able to leak into the water of the system it will cause problems including airlocks and surging of the pump.
The NRV's are located in the divertor assembly. Look to where the Hot and Cold water braided hoses connect. Unscrew these and look inside the dirertor when the hoses were connected and you should see a white plastic NRV held in place by a 'C' clip.