Odd Shower Pump behaviour

[stulish]

I have a Salamanda shower pump feeding an electric shower from the cold tank in the loft. Everything works fine until you turn the temperature up on the shower, once the dial is between 6 and 7 the pump starts to turn on and off in a rhythm (1 sec on 1 sec off - i didn't time it). there doesn't seem to be any air in the system and the system worked fine with my old Triton shower pump. the pump is single impeller.

Any help would be much appreciated.

Thanks

 

[picasso]

Turning the temp up on the electric shower restricts the flow, probably not enough flow for the pump to operate properly.

 

[stulish]

I thought it might be something like that, so would i need a different pump, as the Triton worked fine