Riello natural gas burner locks out?

We have a Riello burner that keeps locking out. it will do the pre-purge, then ignite, and within 5 seconds it shuts down and locks out. Typically when we push the reset on it will run, but we are having to push the reset up to 5 times a day. We have been dealing with this for 2 years now, since we bought the home in 2009. The furnace gy that installed it warranties his work, so he comes by and tries to fix it and doesn't charge us a fee, but he has been trying to fix it for 2 years now. He even replace the entire Riello burner and it is doing the same thing. he has talked to the Riello company and has done everything they have told him, and yet it still doesn't work. We are at a loss and have no idea what to do. We are in maine and it gets very cold at night. in fact, as I type this, we are unable to get the furnacing running and it is 23 degrees outside. We are at our wits end and don't know who to call or who can help us. We have no money to pay anyone anything so that is why we stick with the same furnace guy and pray he fixes it. He has even said he doesn't know what to do because he has no idea why it won't work. Please help.