I had the same problem for a while, it would come on every 80 miles or so, especially if I was cruising around in vacuum a lot.
Turns out that one of my catch can lines was not FULLY tightened. By visual inspection it looked like it was sealed tight, but it was only in a fit of despair and rage while I was untightening and retightening everything did I notice it was not 100% tight.
Moral of the story is, even though a Pre-turbo hose may LOOK like it is nicely sealed, it may be pulling in just enough air to make the CEL go off under vacuum.
Hope this helps.