sync seems to freeze up sometimes

I'm trying to upload 1.7GB to s3, and I keep having to restart the sync, because it seems the client freezes. For example, this is the output I see now:

> 436.7MB/1.7GB MD5, 371.0MB/371.0MB 100% done, 625.8KB/s

The speed on the right is steadily dropping, as though a sliding window is measuring more and more of a period of nothing happening.

The above is the 4th time this has happened during this synchronization attempt. I Ctrl+C and Up Enter, and it gets further each time.