I'm not surprised you are having problems with your Bosch plugs. At 18K miles, it is by no means impossible that they are worn out. When you take them out, see where they were made. Bosch plugs are now made in a dozen countries around the world. I have found the ones from India to be JUNK. Germany is best, Spain is OK, not so sure about Mexico or Brazil.

Bougicord wires (made in France) are OEM, and probably the best choice, though expensive.

Use the little packet of silicone grease that comes with a set of wires inside the plug wire boots to keep moisture out and prevent corrosion. If there doesn't seem to be enough of it in the packet for your unusually wet conditions, you can buy a tube of it at a parts store, and use as needed.

I would NEVER use Champion plugs, especially in anything with an aluminum head, due to the way they are made. The threads are cut into the body with a lathe, and as a result, come to a knife edge. This makes it very easy to cross thread them, and destroy the soft aluminum threads inside the hole. Most other brands of plugs have threads that are rolled between dies. These threads are more or less blunt, and MUCH less prone to tearing up heads.

If you are having problems with Bosch, my recommendation is to try a set of NGKs. They have the good threads, and have always run well for me.