It means you have dodgy plugs, bin them.
I would use Motorcraft plugs, the ones that were th dodgy postions will become dodgy again as the "new" ones start to leak in that position.
This is caused by amongst other things oil mist, so fine you can't see, this forms a path for the electricity to leak out from between the plug cap and porcelain part of the plug, this is why FORD use SILICONE on the plug lead ends inside the cap, this is a VERY known problem, sometimes just wiggling the cap will cure it for a few miles.
For some reason BOSCH plugs are much worse, most likely due to a higher resistance than the ford ones.
tabetha