how can i fix my triton amber shower?

moved into a new house 6 months back and had no problems with shower until recently. its a triton amber and its started running cold all of a sudden half way through a shower. it has always fluxed when water is run or toilet flushed but there is none of that happening to cause it. sometimes you will get a 10 minute shower out of it no probs but more and more it is starting to run cold with no pattern to it. sometimes it will go cold after a minute sometimes 5 etc. if you turn it off and back on it will usually run hot again until the same happens and we always have it on between number 3 and 4 heat rating but when it goes cold if you put it up to say numbers 7,8 or 9 it will be hot but with hardly any water pressure and it is too hot to shower at this rating from the start so cant try it at that level. any ideas anyone?