Yep, it ran better when the engine ran rich on throttle. The fact that you are capable of breaking out of your bracket means the engine still has plenty of potential. I am not sure if the ECU in the 4G64 works this way but some OBD-II's can be forced into 'closed loop' mode by grounding the O2 wire. If it can go into closed loop you can then retune your ignition and make use of the extra fuel. It shouldn't be running that lean AFAIK so there's a chance that something is up
Get a code reader onto it and see if there's a fault somewhere (will be hard to detect if it's intermittent) Check your plugs for visual signs of leaning out. An exhaust leak upstream from the O2 sensor? Could also be a tired fuel pump not providing full fuel pressure or a clogged fuel filter or pick up in the tank. Maybe an injector clean is in order too...
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