Furnace problem: 1st burner ignites, then the furnace shuts down after the 2nd and 3rd burners fail to light.?

....after a few cycles of this, the system shuts down altogether and i have to manually shut off/on the power to retry get the furnace to get going. I have cleaned the pipe out that sends the propane to the burners. I have checked the flue for blockage. I would say one in every ten times, all three burners light. We checked the propane pressure coming into the unit and it's 10, so that should be fine. Any other possible ideas out there? It's cold. I need to keep my family warm but would like to avoid a big bill if there is a simple fix. Thanks.