I fixed it myself (pure genious that I am
I had a feeling it was hitting a character or mapping it didn't like, although the log wouldn't give me any more info.
So, I went through my 600+ contacts and made sure there was SOMETHING in the last name field. T-Mo rep advised same - which sucks because some people I have as "Joe", which seems to be a no-no as they want both name fields filled out. Hence, Joe became "Joe Joe" as did some others, and voila. Five hours later!