Go through all your connections, clean and dielectric grease, including the coil ground. Next, check the coil HV wire at the coil and the plug cap. The HV lead simply threads onto a self tapping stud in the coil and cap. This could be loose and/or have some corrosion from water getting in. Cut off a few mm of the wire on each end and rethread into the coil and cap, sealing with a little RTV.
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