Have Camray Quartet 60-80?

Had above boiler approx 10 years, has been running well, till last year. It was stuck on lock out and would not start up, after engineer checked and cleaned, serviced it, started, ran for week, into knockout again, and not starting up. Engineer found motor had stuck, so had new motor and starter. All went well for a week, now boiler is firing in and immediately stopping again, keeps repeating this, till it has a rest, and starts again, It is very annoyinging and smelly. Please help.