Sounds like you need to replace your spark plug tube seals. You basically take the valve cover off, and pull the spark plug tube seals off the spark plug tubes. I know on my 2009, subaru redesigned the spark plug tube seals. I took some pictures in the link below.
The procedure might be a little different in your 2005 though, I can't say for sure. On the 2005 turbo models, you have to put liquid gasket on certain parts of the head, where the valve cover gasket presses up against. I couldn't really find any specific instructions in the factory service manual for the 2005, it just says to use a new rocker cover gasket.
I think this link below might more closely match what you have to do, compared to the pictures in my link above.
**Just realized I misspoke and thought you had a 2005 instead of a 2006. Either way, I think what I said above should still apply.