If it's not the coils or boots, and the motor is running fine up to that point but suddenly starts to run rough, could it be that there is some emission control device that's introducing exhaust gases into the intake at a time that it shouldn't--like a faulty EGR system (valve or switching circuitry)?

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles