These are all things that can be tested. Look at the "Evaluating Electronic Engine Controls" article in the DIY section of this site. Steve explicitly went throught this for an M103 engine just within the last two or three months, so do a search.
My money is on the O2 sensor.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino