with windows 8 if it a clean install you need java..flash and some time mp4 codex pack like divx to decode some videos. also with youtube hit the pause button on the video and walk away for a few then see if it plays fine. if it does check the flash setings for how much data a video can download on your pc. if it set to low..video will stutter...the other issue will be bad network driver or bad network connection.
Is it a downloaded/streamed video from online or are you playing a local copy of a ripped movie? If it's streamed, might be linked to network connectivity speed or CPU/GPU performance. The higher the native resolution of the content, the more processor and graphically intensive it is.