Burner will run for a couple of hours and then...

Question

I have a Riello 40 burner. I had oil delivered this past sunday. The burner was working fine until the oil delivery(may be a coincidence.) When the house got a little cool I checked and the burner had the orange light on indicating that it had tripped out. I hit the reset button and the burner started up again. I had to do this several times during the day. The burner ran all day monday without a problem but the problem occured again on tuesday. I was finally able to restart it again at 2:17 am on wednesday mornign. Again it ran for several hours before tripping out again. During this situation it took mulltiple hits on the reset button and it still keeps tripping out. Normally even if the tank runs dry it is easy to restart. Now it is almost impossible to start.