Unfortunately, the reason for this is clear. That's because your machine's processor is weak. As long as you watch the video in low resolution, you can still handle it, but you can no longer cope with a video that is viewed in Full HD or even higher in unresolved resolution, for example. To see if this is really the case, start a video that is stuttering, then right-click on the taskbar below and launch Task Manager. There, in terms of performance, you can immediately see that the load on the cpu jumps to 100%, if so, that's the problem.