They stop and don't move anymore? It's common for the dots to move more slowly when you aren't adjusting your aim, but I haven't heard of that. A lot of people have been having trouble recently especially on Chrome, and it seems as if may be related to a Flash update. I and others have had success on Maxthon.
Some recent topics like "Anybody else having issues" and "Very bad lag" might have some more info and/or a solution. At the very least, it's a place to commiserate with others.