I have a 1988 300E, California model. Recently, the engine light came intermittently after the engine is at operating temperature.
The idle was hunting (surges and hesitations when idling and driving) and the engine eventually stalled. When cruising on the highway, the RPM fluctuating as the engine stalled and restarted.

Replacing the Oxygen sensor, over-voltage relay did not resolve the problem. Suspecting problem in the fuel delivery system. Please advise if you have come across this problem.