Last winter I started having some problems with the power steering system on my 05 GT. My symptoms included noise from the power steering pump, bubbles in the power steering reservoir (which usually resulted in overflowing fluid out of the reservoir), and intermittent power steering loss in cold weather when the car was first started. When the car would warm up, the problem would go away as the power steering system bled the air out of the system.
I was able to track the problem down to the o-ring on the feed fitting on the top of the power steering pump. When cold, the o-ring would shrink and let air into the system. When the engine warmed and the o-ring expanded, the air leak would close and the system would return to normal eventually.
I tried replacing the o-ring with something off the shelf and while it did help a little, over time last winter the problem started to return, though not as bad. I knew this was related to the o-ring material not being correct for power steering fluid, but my Subaru dealer said they were unable to order just the o-ring by itself.
Fast forward to this year and I figured I'd try again while I was picking up valve cover gaskets from the dealer. Amazingly, the parts counter guy was able to get me a part number and order the o-ring. Put it in this weekend and this morning with temps in the 30s, I had no issues with my power steering.
It's a cheap easy fix if you have the same symptoms I did.
Subaru part number is 34439FG000.
Hopefully this helps someone else.