Wires were purchased brand new back in September, only 15K miles ago. Same with the cap & rotor. I pulled the cap and rotor, and while it did have some pitting it looked fine so I cleaned and reinstalled. I also checked for tightness and proper installation of the rotor - everything checked out fine.

Should I switch to the Bosch H9DC0 plugs? I currently have NGK V-Power plugs in there; they are listed as acceptable replacements.

Or should I test and possibly replace the coil? I suspect that it is not putting out as powerful a spark as it should. I have done a search on testing the coil, but none of the results are very clear on exactly how you test (i.e. put the meter probes on which terminals) or exactly what the acceptable range of results would be.

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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