Mike (Eliot),

Do you KNOW the two cylinders are running lean/rich, or are you just looking
at the plugs and surmising? There are a ton of things besides the mixture
that can make plugs look whiter in one or more cylinders, eg., certain oil
additives, a water leak into the cylinder, etc. NGK has a site with pictures
that gives a good clue as to what the plug appearances actually mean. I'll
see if I can relocate it.  Any way you could post pics of the plugs?

Your O2 sensor isn't the problem. There's no way it could make only two
cylinders run leaner or richer. I don't see how any other central fuel
delivery component such as the FP regulator or pump could cause two
cylinders to run lean/rich, either. These provide equal pressure and flow to
all injectors, even when they're faulty. They don't choose.

Have you done the Bentley "ECU pin test"? Since it's free, that would be a
good place to start in chasing down your problem. If the resistance in two
injectors is markedly different from that of the other two, it might
indicate a problem along the lines of what you're talking about.

If you've recently run any oil additives, change your plugs and drive a
hundred miles, then reexamine. If you're losing any coolant at all, and the
plugs on one side are whitish, that could explain where the coolant is going.

Geza