I Had The Same Trouble With A 1992 400e. Spent A Small Fortune On It As It Would Not Store Fault Codes. Finally It Was Determined (guess) That It Was The Throttle Body Causing The Problem. I Seem To Remember That Once The Tb Has Been Replaced, The Engine Mgmt System Must Learn To Communicate With It Properly. You Cannot Just Start The Car And Drive It. There Is Some Simple Procedure One Has To Do (mechanic) To Initiate This Relearn. Then All Should Work Correctly.

The Only Other Thing I Can Think Of Is Vacuum Leaks Somewhere (maybe The Gasket At Bottom Of Tb). Good Luck.

JUST REMEMBERED ANOTHER TWO FACTORS ---- BRAKE PEDAL SENSOR & GAS PEDAL SENSOR OFTEN CAUSE PROBLEMS ALONG WITH TB MALFUNCTIONS.