I have two questions:
1) The ignition failures that lead to the lock-outs do not involve any flame. That is, they occur before ignition; the gas just doesn't seem to flow and the ignitor is definitely on. If the flame sensor was the problem, wouldn't the failure occur after the flame was present, that is it would not detect the flame even though one was actually present? Isn't the flame detector's only function to detect for absence of flame when gas is flowing into the burners and to shut off the gas at that point?
2) Assuming it is the LP gas, what could be wrong with it? What sort of adulteration or impurities should be looked for? If it's a problem with the gas, why would it work most of the time and not others? The first failure followed the last fill date by three months. The second failure followed the next fill date by three weeks. Now, on the same tankful, lockouts are increasing in frequency to four or more a day.
I'd greatly appreciate any suggestions on what could be wrong. I'm at a loss.