riello 40 starts, runs fine then stops. ?

hi, my riello burner seems to start fine but after about half hour it stops it sounds like it tries to come back on but wont,however when i lift the gate at the side it comes back on i had an engineer out he bleed it checked the photocell said it was fixed but it has went again any ideas on what it could be as i dont fancy paying to get another engineer out if i can fix it they are way to expensive. pump was also checked and seems to be fine as rads in house heat up when burner is operating.