This time I tried something a little different, and added a coating of silver solder to the electrode. The silver content should be more readily ionized and lower the resistance of the sensor electrode; that's the theory anyway. I also made a standby sensor electrode from stainless, also silver-coated. The problem is definitely in the flame sensor circuit. The electrode is in flame during the entire time the boiler is firing, so it tends to oxidize. The silver will oxidize too, but it should still be more conductive than the alloy that's in the bare electrode. Or something.