Arching SparkPlugs? 2001 Jetta

Ever since I have owned my Jetta (bought second hand in 2004) if the car got wet (ie from rainfall) until the engine heated up, when I was stopped (and the engine was running), the car would stutter a little. As soon as the engine was heated to 190 the car stood still smoothly.

Last year I moved to a new geographical local that gets a lot more percipitation, and this past winter the problem started to get worse to the point where my engine light came on and the car felt like it was about to cut out (while it wan't warmed up). I immediately brought it to the dealer and they diagnosed the problem as arching sparkplugs - they were replaced, and all seemed well. Now, the problem still existed when the car got wet (as in the beginning) - but I assumed that was just a feature of the Jetta when the car got rained or snowed on.

Now, the problem seems to be intensifying again, and my engine light came on for a few seconds (it rained heavy here last night) - but as soon as the engine heated up - the problem went away.

Can anyone tell me how to permanently solve this problem? It doesn't seem right that from now on I am going to have to pay my dealer $300 to replace my sparkplugs every 6 months. Would covering my car work? I live in a very wet climate - so rain/snow/fog is a fact of life.

Any advice would be very helpful! Thanks.