Pop the hood and take a spray bottle with plain water, then "mist" the ignition wires as the engine is running. Do this when it's very dark out. You might be surprised at the arcing you see. This happened to me on a set of ignition wires that were newer and still looked fine. Micro cracks develop in the insulation leading to misfires and occasional stumbles. I must admit this was on a set of Bosch wires from AutoZoo.

Just a suggestion.

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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