glowworm micron 60ff-keeps cutting out?

I've had this boiler 12yrs--when it's on its fine! But it keeps 'going out' with the green light flashing. To get it to start again, it needs switching off. It then cyles though and starts up--but only stays on permanently until after its 3rd relight.
It can stay working for weeks, or not light up every time its needed. My plumber has tried everything..any ideas?
PS the solenoid is quite noisy.

Many thanks if you can help