Myson Velaire going to lock out?

I have a Velaire 40/50 oil boiler that works sometimes however after it has been off for several hours when it trys to light it fails and goes to lock out. Once the reset is pressed it lights OK and keeps working. When it fails I hear the pump start and I hear the relay click over as it appears to try to start but then it goes to lock out. It does not do it ever time but about 50% of the time.Any Ideas?