So, BWS..... I wonder if you would mind providing more detail on what you saw regarding the wasp nest at the igniter. What is the 'it' in 'Cleaned it off'? Are you saying you removed the igniter/flame sensor assembly and found the electrode rods had a mud nest across them? So was it likely an electrical short rather than an air-flow blockage?
Also- in retrospect, were there any subtle clues it was there? (I guess I'm assuming it's a mud nest and the combustion-air fan may have pushed small bits of mud and/or insect remnants into the exhaust tube where they'd be visible for troubleshooters to realize, 'OK, time to pull the igniter.')
-OC
PS- Oh, yeah..... And what model furnace is it?