Intermittent oil flow causing lock out?

20/25 Worcester Bosch boiler 15 yrs old regularly serviced went on lock out last week, worked fine till then. Air in oil pipe appeared to be problem, but after numerous engineer visits to
Change oil pump
Change Tiger Loop
Change flexible pipes to burner
Renew oil feed pipes (2.2 mtr lift and 15 Mtr run )
The boiler will only fire for about 30 secs before lock out.
It will run constantly with direct oil feed from a container at the side of the boiler, but not when connected to the main oil pipe work
Any ideas on why intermittent oil/air being pumped into boiler, and possible solutions
Thanks
ken