What could be wrong with my Glow-worm micron 50ff ?

Within seconds of my boiler "clicking" to come on the boiler then "clicks" to go off, this can happen twice or three times within the space of a minute, it will then "click" on and run until temp is reached and will then "click" and go off until the next cycle, the boiler is 7 years old and last had a service 2 years ago, any help would be appreciated, its still summertime and i would like to get the fault sorted before the winter, thanks all.