Pop the hood at night in a very dark location, perhaps unlit garage (with the door open of course). Look for any arcing of spark around the cap and coil, but especially along the ignition wire. Spray an ultra-fine mist of water along these areas to see if doing to creates any arcing. If you get lots of arcing, replace your wires. As noted earlier, the cap and rotor are also usual suspects.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280