John, Well that's good news.
Sorry for late reply, painting the kids' house today.
Yeah, I think the Calif version will read 85% until it goes closed loop (the 49-state reads 50% unitl it warms up). If you have the LED on the
Anyway, if it is oscillating around 50% +-10% fully warmed up, then you have a lot of things working right. This indicates the O2, EHA, fuel pressure, air valve are working pretty good, cause it's checking and controlling fuel mix just as it should.

I know we discussed head gasket, and we ruled that out, due to the symptoms. And the way the problem came on suddenly, then healed with new plugs, for awhile, tends to point to something non-mechanical that failed.

My thoughts for the day:

Drive it around and see how it does.
Get a piece of speaker wire and splice the meter to it, so you can drive around with the duty cycle meter on the dash. Might see something under load; particularly as it gets nice and hot from driving. Maybe a sensor is going out under temp. If the duty cycle holds good though, that should mean fuel delivery is not causing it to run so bad, correct?

Now, I'm really thinking the code 4 may be an artifact caused as a result of something else going bad - since the duty cycle is right on, seems the air meter must be doing OK.

If it's not fuel, it's likely ignition, if it comes and goes, might be coil or ignition component? Maybe that's why the new plugs helped for awhile.
Might try to see if heat affects it, and if when it runs bad, what happens to the duty cycle.

DG