Vaillant Turbo MAX 282E?

Hi i have a vaillant Turbo Max 282E i have a problem that when i open the hot water after 3-seconds it goes cold and when i turn the tap of and then open it on again it comes warm but same problem so i had changed the diverter valve but still same problem so then i de-scaled my Hot Water Heat Exchanger and i still have the same problem and know when i put the heating on it comes on and when i open the hot water tap and when i close it the boiler does'nt turn back to central heating till i restart the boiler by turning it off and then back on.
ALL HELP WOULD BE APPRECIATED THANK YOU.