i have a problem with my hotwater miniMAXX wr11-2.b?

I Have a Junkers boiler minimaxx WR11.2.B. it runs on bottle gas, it was fitted last Aug 2011. When we was dishes or run the tap in the kitchen it's ok but when we shower it's hot to start but 5mins in to your shower it goes cold, I have to turn the tap off then back on and wait for it to get hot again. Can you help please.