This site is a collection of information on a problem with Half-Life 2 that has plagued many players of this otherwise excellent game. This problem is characterized by a sudden massive drop in frame rate when turning corners or moving into new areas of the game. These drops are usually accompanied by looping or stuttering sound during the frame rate drop which is how most people become aware of the problem. Aside from these isolated drops in frame rate, the game is otherwise perfectly smooth. These sudden drops kill the immersion and lead to a very frustrating experience. Below you will find additional details on this problem, some theories, and all responses from Valve on this problem.
We're working on it. The stuttering is actually three different bugs. Two of them are fixed and should (hopefully) be out tomorrow. Still working on the third one which only happens on NVidia cards/drivers.Gary has several more posts in that thread, view the whole thing here.
Gary McTaggart (Valve)