Mercedes: 1993 Mercedes 300E ignition coil?, exhaust stroke, auto parts store

Question
I have a 1993 Mercedes 300E that failed inspection but I was not provided with screen information just "pass" "fail" certain things.  The problem I am having is now twofold.  When I am at a stop sign, the car's rpm will drop down and car will almost stall and then rpm's will rev up and it won't die.  That happens 99.9% of time.  I had one mechanic believe it was ignition coil but didn't have testing equipment to verify so I went to another mechanic that did replace the ignition coil with a used part and now it seems that while stopped it won't do it immediate but less than a minute will repeat prior behavior.  In addition, now, when I accelerate it is making a "knocking" sound and pops when I hard accelerate and almost feels like it won't start.  Car runs great once over 30 miles per hour.  I called mechanic and he is going to replace with new ignition coil.  He was trying to save me money which I do appreciate.  Spark plugs are good, had vaccum leak checked.  Any other ideas or do you agree that it sounds like ignition coil?  Thanks for any insight you might provide.

Answer
Jennifer,
Does this car have a carburetor or is it fuel injected?
I would get some Techron fuel treatment in the black bottle  at the auto parts store and pour it in the fuel tank and fill up with premium fuel.
During hard acceleration, the engine is under load and needs good spark to cause a clean fuel burn. I would expect that you have a cross fire in the plug wires or a spark jump in the distributor cap. The detonation happen when the plug fires before the piston is in top location. The premature explosion meets the piston on the way up and causes the knocking sound. If the plug fires while the piston is on the exhaust stroke then you get a popping sound.
If thee spark is not at the plug on time and hot then you get misfire.
Also, if you have taken on some 'bad gas' it will cause some of these symptoms. The new fuel with alcohol is very susceptible to moisture as alcohol is anhydrous. Absorbs moisture.

Some time the spark leak will show up if you raise the hood in total darkness while the engine is running. Have some one sit in the vehicle with it in park and with the brake set while you watch as they accelerate the engine. If you see blue lighting, then you have located the arcing.

Let me know what you find out.
Good Luck
Robert