I realize that a lot of people have this problem and probably the right place to ask help for it would be Ubisoft's forums. But previously I had no issues with the game (not even the usual issues concerning framerate and graphics other AMD users seem to have with Assassin's Creed Unity) and it ran perfectly.
However, once I downloaded and installed the Crimson Edition, this issue has surfaced. The game runs for a variable length of time, sometimes a few seconds other times upto 15 minutes, then freezes for an instant and finally crashes with the message, "ACU.exe has stopped working." I did notice that it sometimes crashes after I do a certain action in the game like speaking to a group of people.
I tried uninstalling and reinstalling both the game itself and the AMD drivers but it didn't work.