The boiler wasn't firing yet again tonight. Since I had just cleaned the igniter/sensor again last night, this was concern-making. I took the unit apart again to get to see the igniter while it was firing.
A couple of things I noticed: the igniter was covered with oxide again, and the pilot flame was only hitting the tip of the main igniter rod.
It's pretty indicative that I need to replace the igniter/pilot assembly.
The other thing that I discovered is that the pilot flame continues burning while the main gas valve is on. and if the pilot flame goes out, the main gas line shuts down too. This explains more why the oxide forms on the electrodes-- they're sitting in flame the entire time that the boiler is burning.
So realistically, the three ways that I could make it work better would be to:
1.) find some way to adjust the pilot flame so that it hit more of the sensor rod
2.) plate the sensor rod and armature with silver (or possibly gold)
3.) get a new sensor/igniter/pilot assembly.
Number 3 is the most reasonable, particularly if the pilot orifice is clogged and is keeping the pilot flame from being as strong as it should be. It's possible that I could adjust the pilot flame at the gas valve itself, but I don't want to do that unless it's absolutely necessary. There might be a way I could clean the orifice (compressed air and a small brush), but a new one would probably work the best. I can keep the old one for a spare.
Or I could hack the old module and get it working with a lower sense voltage.