shower suddenly running cold?

I dont have central heating my coal fire heats my hot water.I have an electric shower and over the last six months it has strted running cold water every few minutes while it is on,to begin with it only did it every five minutes or so but it has strted doing it every two minutes.I have taken the shower head of and cleaned it but nothing is helping.Not sure how old the shower is.Usually you hear a click just before it runs cold.Model-Triton t80i