The pack file has a corruption...

My pc has crappy cooling atm, so sometimes it shuts down during play.. (not good I know) and my pack file will get corrupted. I also live in New Mexico with the worlds worst internet.

What I do is simply back up the file after a patch and move it to a different location. Then if it corrupts I just restore from the backup instead of having to download the full game again.

Hope this helps.