In reading though your post again, I thing I noticed something.

From the way you wrote the post, I believe we all presumed you were playing back the audio of the file on Catalyst.

Reading again now, I don't believe this is the case, right?

You're just playing the video portion along side the audio coming from another system, right?

If this is the case, a test you can do is to enable audio mode on the file and run it with the timecode from the audio dept and see if the audio stays matched near the end. If it doesn't then it means the file you're using is playing at a different time base.