I had this problem.
The main crank pulley is actually made of 3 pieces [pressure?] fitted together, a center metal pulley, a rubber (i think it is) ring called the harmonic dampener, and an outer metal ring. Check to see if the harmonic dampener is sticking/slipping out. It happened on mine and caused the pulley to wobble when the engine was on. Took it in to the dealer and they fixed it under warranty though.
Click here to see my thread on wrxfanatics.com
There's a pic of it too.