Check your harddrive performance. Open task manager and click “performance” and look at the one that says “HDD” or “SSD.” If it is running at a constant 100% then it means your drive has failed and you need to replace it. I’m just going out on a limb here though, because other HDD failure symptoms include general sluggishness with all aspects of the computer though, not just videos. Other recommendations would be updating/reinstalling graphics drivers, making sure that your temps are okay, etc.
Where are you watching these videos? Does this only happen on YouTube or can you play videos you’ve downloaded fine? Do games run fine? We need a little more info.
Do you use Wallpaper engine or anything like that?