I had the same problem. The chances of #5 fuel injector failing rather than
one of the other of the 6 cyl. is slim.
the answer is moisture coming under your hood, leaking past the seal for the hood and ending up on #5 spark plug.
Just stop the moisture from dripping onto #5 spark plug and you are good to go.
I wish someone had given me this advice before I fought the problem with money.