Framepacing. The monitor adapt itself to the length of each frame. This is ok as long as the frame does not last more than 1/30th of a sec, which is the limit for gsync to work. More, you will get stutter.
CPU halt and streaming stuttering or bad frame engine (for example, frames do not arrive in the good order to the monitor) : no solution.
That's... uhh... the definition of framepacing.