You may not want this information, but here goes. I bought my '87-560SEL 12 years ago (28,000 miles), and still under warrantee. The O2 sensor would come on after driving for 35 minutes. The engine would go into "limp mode", but would run properly after shut down and restart.
The final diagnosis was a faulty computer that would improperly reset the engine, thus lighting the O2 sensor.
The computer was replaced (under warrantee), and no problems since; 156,000 miles and still as strong as "new". Hope you find a cheaper solution.
1987 560SEL (156,000)
1983 300D Turbo (225,000)