Intermittant Lockout ?

Hi I have a Worcester Danesmoor 12/19 wall mounted boiler.
My problem is intermittent lockout. The boiler will run fine for a week or a month or sometimes only a day.
I then press the lockout button and normally it fires up fine but recently the pump motor runs for a few seconds and the light comes on again and it does not fire up after three or four presses. I once cured this problem by removing the burner control box and cleaning the contacts and it also started after pressing the burner control lockout button.
The boiler then runs fine until it happens again. Any ideas?