Thanks for your informative and quick response, Greg.

Your description of my furnace cycle sounds exactly like what is
happening. I don't know, however, that I have never noticed a spark
ignition at the point the gas valve opens--even when it is functioning
normally. I do have a distinct orange glow, though, that seems to
describe a "hot surface ignition." That glow subsides at ignition or
ignition failure.

Gas pressure has been measured both at the point it enters the furnace
cabinet and inside the valve. Both pressures have read normally.

Is their a way to isolate whether or not the intermittent failure
occurs from the gas side or the spark side?

Thanks.

Bill