Find the source of the A/P activating. When the A/P comes on, it also opens some valves to let the air into the exhaust system. Any air leaks into the exhaust will cause extra oxygen to get in and fool the O2 sensor. I have seen poor running on a 420 that turned out to be one of the hoses hooked to the air pump leaking causing air to get into the exhaust. I think in your case, when the A/P is activated, the switch over valve (i think that is what it is called) is also activated causing your problem.
There is a temperature sensor somewhere (I'm not sure which one it is right now) that allows the A/P relay to turn on to activate the A/P.\

Look into this and let us know what you find.

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84 500 SEL (307,xxx miles)