I own 2008 Legacy GT and had the same issue (rough idle and flashing warning lights in cold temperatures) discussed in this thread for three consecutive winters. Visited Subaru dealer at least 10 times for diagnoses/repairs but the issue kept coming back.
Finally, last December (2013), I printed parts of this thread, brought it to the dealer and asked them to read it while I was waiting, which they did, and after my "gentle push" they agreed to change the gaskets between the plastic intake manifold and the tumbler valves.
It appears that this fixed the problem, as the issue has not reoccured at all, despite it being an extremely cold winter.
Thank you everyone for contributing to this thread, and especially to @bmx045, who correctly diagnosed the problem.
