How do I stop my grant euroflame 50/70 from locking out?

I have a grant euroflame 50/70 (oil feuled boiler) and when the boiler reaches temperature it shuts off fine but as it cools down the pump starts up again but the boiler won't fire up and it goes into lockout. The only way I can get it going again is to take the photocell, reset the over heat button put the photocell back in and then it will start, but the same thing happens after it reaches temperature. Any ideas what the problem could be!