Some STLs can be too high in resolution. The resulting gcode can be very large, and as a result even small movements of the machine might be made up of many tiny lines of gcode. When this happens, the amount of information that is being sent to the machine might be greater than the USB cable can handle or more than the Atmel Chip on the Arduino inside the machine can stomach at once. Sometimes, your machine might even become unresponsive.
The machine deals with this sometimes by stuttering - sometimes even for several seconds - before continuing. The solution is to export your STL to a slightly lower resolution. Anecdotally, circles will still come out great if they're really just 360 sided polygons.