Is your car running rich? My cousin just got done troubleshooting a very similar problem on his 93 400E. The problem turned out to be a bad fuel pressure regulator - his fuel rail pressure was over 100PSI. This high pressure was permitting too much fuel into the engine when the injectors opened. The result was a rich-running engine that did not idle well and stumbled on acceleration and sometimes a no start condition due to wet spark plugs.