Similar story to everyone else - I leave it run and come back to find that 68 or some random number of songs upload and it's frozen. Restart the process and it will upload another random amount of songs. Then, strangely, yesterday, it uploaded 1200 songs, and I thought maybe I was going to get this thing finished with by the end of today. It stopped again shortly after dinner - 4 songs uploaded, got it started again, stopped, repeat lather rinse. Woke up this morning to find that it had stopped sometime shortly after I went to bed, another 60 songs uploaded. I also noticed it would stop and start itself! When I got home yesterday and saw the huge number of uploaded songs, I stepped away, then came back and it was just restarting step three again. Weird.
I wish it would finish partially uploaded albums first. I've got albums with one song missing, but other albums completely upload instead. Would love to know what the algorithm is for determining what goes when and why.