High clorine content in your brewing water is the most common reason for a plasticy off-flavor. Do you happen to use tap water without filtering it, or have you recently cleaned any of your equipment with bleach?
The other would be something plastic used in the brewing process that is not rated for high temps and is leaching plastic.
I'm guessing it's an off flavor produced by spices boiling.
I've never heard of boiling spices causing any off flavors. I live about 3 blocks from an organic market, and use alot of random spices on a whim, especially in Hefeweizens. I always add these as 5 min additions in the boil, and have never had an issue.