Worcester 3.5 high flow boiler?

I have a worcester 3.5 high flow, it is about 18 years old. The boiler keeps cutting out, sometimes several times a day so the pilot has to be relit.

I have had several engineers out who have replaced several parts, gas switch, expansion tank, thermostats etc. It has cost me a small fortune in repairs and the problem still persists.

Has anyone got any ideas as to what could be the cause of the problem or could they point me in the direction a competant engineer.