Shower pump problems?

just fitted a stuart turner pump as per instructions, on a mixer shower.the pump is a single connected via hot water, the cold is fed by gravity from the tank in the loft, heres the problem when the shower is set at the right temp to shower it pulses on then off, if i turn it to red hot the pump works fine, when i turn it down to cold it cuts out, the pump will not run at the right temp to shower, so i took the shower head off and turned the shower on, got it to the right temp it worked fine until you put the shower head back on, it starts pulsing, off i went to b and q and bought a new shower head for power showers still the same, please help its doin my head in now, thanks pete