how to stop the power cutting out on my Mira Advance shower?

how do I stop the power cutting out to my Mira Advance shower? You pull the cord to switch on the shower. You press "High". It runs for approx 5 minutes then cuts out, without any lights on the shower box itself showing! Many minutes later, it comes back on but with cold water first. At no point does the red light on the pull cord turn off, it's just on the shower box itself. I make sure there are no other water demands such as loos, washing machine etc but it is erratic and annoying! Please help!!