I had this problem when 1.10 came out. I wouldn't get CI, but the game would freeze, and I'd have to Ctrl Alt Del and restart it. I wrote an email to Blizzard about it, and they never replied but the problem went away.
It happens only the first time you enter a new act, right? Any further times it works without any problems?
The first time you enter a new act you normally see a video with the story going on. Some system configurations or card's obviuslly have troubles with it.
Maybe you only missing the codec.
This happens to me whenever I first complete an act. I think its just a form of lag. After he finishes an act, your character will appear in the next act, but you have to watch the cinematics. Even if you quickly exit out of the cinematics, you still have to go through another load screen. I assume the game thinks, while you are in the cinematics screen and the next loading screen, that you are actually lagging out, and therefore you get booted.