I hvea friendly format 100e that keeps stopping and starting up and ?

It fires up and runs for about 5 minutes then cuts out for 5 minutes then again fires for a few minutes then off again. It does this all the time and my flat rarely gets above 17 degrees. I have had new room stat and all tests on pressure etc on boiler comeback fine. Its doin my head in lol. Can't get my flat warm. Any ideas?