this sounds like a fuel pressure problem...either with the pressure regulator or the actual fuel pump is going bad..

follow up - I had checked my fuel regulator in the past, but it did not appear to be leaking. Checked it again recently and sure enough, there was fuel in the vacuum line that connects to the regulator. I changed the fuel pressure regulator, and she now starts on the first crank every time - hot or cold. That was a week ago, I'll post if there are any updates.

Feburary, did you ever find a solution to your problem? The symtoms you described are an exact match to my problem.

Thanks in advance for any help

February, did you find a solution for your 400E's running problem(s)? They sound very similar to problems I'm experiencing with my '92 400E. Thanks.
-Forrest

Finally Fixed:

My INDY solved the problem:

Bad Fuel Pressure regulator and fuel injectors.

A VERY long Drama but email if you have any specific questions.

thanks

-andy

Cool. I suspect the same are responsible for the somewhat shaky starts and occasional low-power feel my M103 has....

misfiring intermittently
 

I had the same 400e problem. After many months of checking it out by dealer, it turned out to be a throttle control unit. Pretty expensive but they become bad on 400e model. Also check out if one of the injectors is intermittent which I had which gave same symptoms as intermittent missing.

misfire hot
 

I JUST FIX A 95 S420 CAR STARTS COLD. DRIVE CAR FOR 5 MIN MISFIRE STARTS WILL NOT RESTART HOT MUST COOL DOWN ABOUT 30 MIN.
I FOUND WIRES TO CAM SENSORS INSULATION DISINAGRATION MAKING CONTACT WHEN HOT HOPE THIS HELPS

m119 95 E420 misfire
 

I have a 1995 e420 w/140k miles which I purchased in late Sept. On the 2 hour ride home the car ran flawlessly. It did not reach the optimal temp (80-85) on my test drive, (1 hour) or on the way home so I knew it needed a new t-stat.
I figured I would drive a few days and make a list of todo’s, more extensive than the original test drive list.
My first day commuting everything was smooth sailing until I hit bumper to bumper traffic. Upon creeping the engine temp remained around 60, and a misfire commenced anytime I was creeping. As I accelerated I smoothed out. It only occurred with the engine was warm.
I consulted the web and found that many people have experienced the same problem with many suggested solutions. I knew it was either spark or fuel delivery related.
I have the unfortunate advantage of recently totaling my 92 500E. There are many parts sitting in my garage, including the venerable m119.974 5 liter. I swapped many sensors (CPS, coolant temp), and distributor caps and rotors. Nothing resolved the problem, until I swapped the fuel pressure regulator (FPR) , Viola!.
As someone mentioned, it the best 2 minute fix ever completed. $40 bucks a ********AZ or ********.
Here are some DIY links part # included. Thanks for all the heavy lifting.
Photo DIY-Fuel Pressure Regulator R/R M119.981 Engine - Benzworld.org - Mercedes-Benz Discussion Forum
HOW-TO: Replacing M119 / M104 fuel pressure regulator
If the regulator is not your problem Good luck. Check the link below.
Everyday Diagnostics: Mercedes S-Class Misfire