I have recently installed a power shower with a stuart turner pump ST55. In the shower itself I have a thermostatic mixer from which the shower head is connected. There are different settings on the mixer but I have mine set for 38 degrees. The problem that I am having is that the hot water is not staying at that temperature for the duration of the shower. The temperature decreases and I find that I have to adjust the setting during the shower to increase the temp. Any advice would be great!