ACM: Firstly, I hope you installed the correct spark plugs in your car, only use what your owners manual specifies, none other. .032 should be the gap. Never use a platinum multi electrode plug, they are mostly all resistor design and you probably have resistor equipped plug caps on your wire set. (metal caps) You dont need two resistors on each plug! Take the last HT plug lead nearest the firewall and put a new test plug on it, ground the plug to the cylinder head and do this in a darkened area. Your spark should be whiteish blue and not redish orange. The little coil body that MB uses in these cars rarely last more than 10 years especially in rainy areas. You will find it better to order a new coil from www.*************** for 66.00 bucks, and you should also change the OVP relay. (this coil comes in a left and right side mount, so check yours visually before ordering) Only the best of electronic technicians can refresh that relay, and if you dont clean it up right, you'll add more problems for yourself. If you can get it out of its aluminum case without breaking it: 1. Clean and Reflow all solder joints, 2. Burnish both contact point surface and blow out grit, (very hard to get to) 3. Deoxidize base connector pins, 4. Retension fuse holder contacts, 5. Test resisitance through the newly serviced contacts with a digital VOM, 6. Reseal and water proof encasement, only then are your good. If not, $60.00 for a new OVP sounds much better. Let us know how you make out.....
Dead Giveaway to your problem is: Car ran better after wire change, then started running bad soon after. Ignition coils get gradually weaker and weaker.....
Dave...

"93" 300TE 4 matic 187,000
"85" Volvo 740 Turbo 166,000
"85" Honda Limited Edition Gold Wing 1200 17,000 miles