If your CS, or CSS, or some other Valve game is choppy, it could be the evil Steam engine thieving CPU from you. It doesn't matter what CPU speed, it can happen randomly, with Steam using 99% of your CPU while you play. This doesn't happen very often, but when it does, it's really quite annoying. All you have to do is lower the priority of Steam to your computer. To do this, right click on your taskbar (the bar with the start button), and select "Task Manager".
Right click on the Steam.exe process and select "Below Normal", like so
If you'd like to add to this tutorial, please comment.