Worcester greenstar junior 24i cutting out?

Hello i really hope someone can help.
I have a Worcester greenstar junior 24i boiler, and even though the heating works alright, it seems very low temp even though set at max. Also, when running the shower or bath, it will run hot for about 10 mins then run cold. while running cold the boiler emits a clicking sort of noise, like it is struggling to ignite. The blue power light remains constant and does not flash at all to indicate any fault . the pressure is in the green. i was wondering if any kind gent here had any suggestions as to what the problem could be for me, many many thanks xx