Have installed a thermostatic shower with a twin impellor pump. The pump has an independent cold feed from storage tank in loft, and hot feed from hot water storage cylinder (both 22mm). When I run the shower on cold, or set to quite hot, the pump works fine, as soon as I turn it to a tepid/warm setting the pump pulses. Asked manufacturer for advice, they suggested raising the reed swithes, have done this to no effect. Any suggestions?