Could have a cracked lead or coil casing or a casing on a connector, with the motor running run a hose gently over all the different electrical connectors giving them a wiggle and time to get wet to see if it repeats itself, then replace or seal it with silicone after you've dried it.

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Beaten up old 98 Gen 2 Liberty, 13.0, stock V3 STI running gear.
07 Transit motor home
69 VF & 70 VG Valiants needing full restoration
Yamaha Zooma 50cc rocket hopefully going with the Transit.