It sounds to me like the entire left (#2) ignition controller is about to give up. This is a common issue with 120s - and almost always causes the other controller to fail with it. It can worst case take out the motor by firing early and bending stuff. I've seen at least 3-4 120 motors that have failed in this manner.

Is Bret from San Diego still hanging out nearby? He's one of the few people in the U.S. that can keep a 140/600 going - he'll know what this is right away.

And SteveB is right - they ALways report low values (it's analog). I guess it's the temp differential is wrong? Temp heat/resistor weak? But this is what I don't get - if they report lean (too much air in the mix) then it should compensate with more fuel. Right? To richen the mixture it adds more fuel at the injector.

Also, a lean mixture will overheat a motor in a hurry, and probably burn valves and o2 sensors.