Ah hah!
So, I bypassed the limit switch and the 2 flame rollout sensors last night as they are on the same loop. It was still exhibiting the problem so those were not the issue.
Standing and observing the unit upon firing, HSI lights up, gas comes on...but not consistently and "cleanly" in the lowest tube or port where the flame sensor resides (see attached image). So the flame sensor is not bad, but its not sensing flame either since there's none there, intermittently. The 3rd time it tried to refire, I happened to give a couple quick blows of air with my mouth into the lower port and it fired up (blowing from a safe distance of course).
So- I am guessing whatever that jet or nozzle is I've circled in the bottom pic is bad. Do those go bad? When the furnance is running its pumping heat, so I guess I don't
think it's low gas pressure at this point, even though that misfiring port is the lowest tube away from the gas input off of the control valve. But...
How best to clean? I can pull it apart this weekend. Unfortunately I don't have a gas pressure tester.
edit-> I should clarify, the top photo was snapped within about 4 seconds of the unit firing and then the control cut the gas flow as it should.
