What's the performance like?
I had this code a while ago (with a collection of others,) it left me alone when I sorted out the maf code (I've now moved my open cone away from the maf meter a little, and it seems happier)
Maybe your maf reading is out,
As the man from badger said, you may have a boost leak?
That valve (pcv I think) that was loose initially. May be faulty? Also, the other little valve under the inlet manifold (to the right if looking front on) can cause this I think, they're only about a fiver...
Also, any little splits or loose breather hoses may allow un metered air in...