Originally Posted by Derf00

You are contradicting yourself. You want to be frugal yet you're already throwing parts at it hoping to fix the problem (spark plugs). What makes you think it's a plug?

If the problem gets worse when it rains I would go with wires before plugs. Plugs would present the problem all the time and not get better or worse when it rains/doesn't rain.

I just noticed this is a 3.8L. Plug wires are a big culprit on these. Go with Motorcraft wires, not aftermarket. Aftermarkets tend to have incorrect bends on the boots which puts them right on the exhaust manifold and burns them out quicker.

Spray bottle trick on the wires will help find the problem wire(s).