Long shot here, but I hear that both Bougicord and Bosch have changed the connection type at the plug over the last few years (have no personal experience). All of min connected using the threaded stud on the plug while the new ones us the nubbin (technical term) that threads onto the stud. Is it possible your new wires and you older plugs don't match and the wires grip just well enough that no leakage occurs until such time as the humidity allows arcing.

Mike