what can i do baxi 3 50 pf?

I have had the plumber out 3 times in recent weeks, first of all the fan was found to be slow, but the heating came back on. then another fan was fitted. then a few days later it went off again. He said it could be a circulation problem we left it on constant, but still it would turn off at night, as previously programme. then a day ago it went off again. the overheat light has come on and the boiler and fan lights are flickering faintly. serial number 01-025343-243098 01-01