Warmflow 70 Boiler Problem?

Hi All
I hope someone can help me.
I have a warmflow utility 70 oil boiler and a riello rdb pump.
The boiler cuts out every so often and I have to reset it. The first time I reset it it cuts out again after 5 secs or so but the 2nd time it works ok for a couple of hours.
I have bled the pump and also cleaned the photo cell.
Does anyone have any idea what the problem could be please?
Any help would be appreciated.
Many Thanks
Dave