Check the wires to see if you may have damaged one while placing the spark plugs. Also check that the connections are tight. I replaced the plugs only on my 190E 2.6 about a month ago and the car ran terrible on start up. It was idling rough and missing under acceleration. I went back and checked all the connections and all seemed tight. It still ran rough. I played with the connections by removing and replacing them and the next time it ran fine.
Also check the wires, for cracking, wear, etc. The wires on these cars are solid core with resistor ends, an in my opinion the ends are sensitive to being handled.
1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T