I'll try this one more time. I mistyped in the first place, and subsequently. In the mean time, I've removed and properly gapped the igniter electrode, burnished the contacts and centered the igniter in the tube.
What I did not make clear originally, is that the unit fires immediately with a strong flame but shuts down after a few seconds (it appears that the gas valve is shutting down). Only on the 4th or 5th try will it remain on. Hopefully that explanation will lead to a better diagnosis. Thanks again in advance.