WTH is wrong with YouTube this past week?

Note this has been happening on several different computers, so it is not a matter of there being issue with the computer. So the issue is that when I'm watching a video on YouTube, namely longer ones, like ones that are at least 10 or 15 min or longer, if I watched most of the video , but try to go back to an earlier time in the video, it starts freezing up and won't play smoothly unless I refresh the whole page. Is this happening to anyone else and also, does anyone know if there's some way to fix this problem- it's getting really annoying? Thanks.