Or maybe plug wire routing? There is a corona that forms around them when the cylinder is fired, which can short to adjacent wires or to the block. Are they held fast and have you routed them so they don't twist together?
Could be a problem that is worse in the winter than summer. Remember electrochemistry, fresh water does not conduct well but salt water does. What makes up road spray in the winter? Magnesium chloride is a salt and makes an electrolyte when dissolved in water. Might even be fine in the rain or water crossings but the increased ionic activity puts whatever problem, cracked cap, failing wire, etc. over the edge.
'91 Pickup - Imelda
'08 Tacoma TRD - Donna