Are you able to get about 44C from the shower when the flow control is turned down. If so then the shower is working normally. The alternating hot/cooler water ouput is caused by the shower thermal cycling on the thermostat. This is normal. Electric showers do have quite a low flow esp. this time of year when mains water is very cold! maybe about 5 litres a minute in this weather.
garryk
December 2009