Use a different Uploader if possible.
I've found the Java uploaders, here and elsewhere, to be quite trouble-prone on my Vista-based PC. I would imagine it's no different for Mac users.
The quality of your net connection can also have a major effect on how well an Uploader works. If more than a tiny handful of data packets get dropped or corrupted, by the routers along the way, it will cause major uploading problems.