Silly problem. I have the same PCM-3350 on 6.3.0 (and previous on 6.2.1) and I did not have any network issues at all. Is the cable (from the PCM-3350 → RJ45) ok?? The cable is very fragile, maybe the cable is busted or wrong.
Try to fix the cable, try a new cable or try the original cable if you are using a home brewn.
It could not be more silly. My cross-over cable (I have been using it several times without any problem was wrong now).
Fmartens, after you have told me that you have used the PCM-3350
network card without any problem, I have then started to think that I could have a problem with the cable and not with the configuration or the drivers.