How can I fix my Glow-worm 30cxi - hot water goes cold when having a shower?

Hi,
I've had this 30cxi model for about two years now and have had no major problems apart from the pressure dropping a few times but all ok in that department.

the problem I have now is when I have a shower which is basically a showerhead connected directly to the main bath taps, after about 1 or so, the water goes cold then after about another minute, the hot water comes back. this cycle repeats itself every 1minute or so. What I have noticed however when showering is that all my radiators are also very hot even with the radiator dial/switch on the combi-boiler is OFF. It's as though the how water which should be only going to the Shower is also feeding the radiators. Never had this problem before and have recommended this boiler to friends as the shower pressure is awsome - not so much now as the hot water comes and goes - but the radiators seem to be siphining the hot water which should be for the shower. Any ideas ?