Since no other suggestions are comming through and you seem to have covered all the basics, I'd go through this.

http://www.simple-talk.com/sql/performance/finding-the-causes-of-poor-performance-in-sql-server,-part-1/

http://www.simple-talk.com/sql/performance/finding-the-causes-of-poor-performance-in-sql-server,-part-2/

When filtering the results go by CPU and you should find the top cpu hogs in the system. Hopefully that'll give you an idea of where to start digging your way out of this one.