How can i fix my worcester 9.24 electronic ?

When i switch the heating on, the boiler will fire up and run for only a few minutes then stop. If this happens, then if i turn on the hot tap for about 10 to 15 secs. to ignite the boiler again, then it stays fired up ok for a short while and then i need to do the same. Sometimes it works ok, but when it is i don't think it is cutting off when the temp. is reached, as it seems to run for a while and i am manually switching off.