triton rapide 4+?

had a triton shower fitted rapide4+ 8.7-9.5 kw approx 16 months ago but for the last three months ish it has been running a little strange you could be in the shower and it would go cold there is no other appliances on draining water if you leave it a day or so and try again it operates fine then will start to go funny again i am not sure of gaurentee because we bought it from focus before they closed but was installed a bit later please help i cant really afford to replace it or pay for plumbers as i have recently become another victim of the recesion