vokera linea 24 hot water cuts out! please help a.s.a.p!?

hi. i have a vokera linea 24 about 9yrs old, for a while now when you have a shower the water goes cold after a few mins after about 15secs its back on then off then on and so on, it doesnt seem as bad if the ch is off, just before it cuts out the boiler starts to make a pressure surging sort of noise and the pressure gauge flickers up and down slightly?

any advise or help would be great!

thanks
andrew