How can i fix my worcester 28i boiler?

Our boiler was fitted in 2005. No problems until beginning of this year. New circuit board put in ran fine. Two months later and boiler has conked out three times in the last week. Plumber tried another circuit board but a few hours later it conked out again. Someone from worcester came and put in new fan, new pump and new power board. He said to check electrics which we did before we switched it on again today. After two hours it has gone again. Can anyone help us? This is driving us mad!!!!