How can I diagnose an intermittent furnace ignition failure?
Ended up having a technician come out in fear of the intermittent failure becoming permanent in this cold winter.

Issue with this old furnace turned out to be that the gas valves were dirty blocking gas flow to some degree. The technician disassembled and cleaned the valves and went on his merry way. 

I learned this is a good yearly maintenance for old furnaces like this.