I am having the same issue too, my youtube seem to buffer alot on HD, but if you turn HD off, it buffer fine. My speedtest are over around 20mbps down and 3 up. The weird thing is my family other nexus does not have this buffering issue on HD. I test both the nexus at the same time and each separately, my is loading slow and buffering. I will test the apk you provide.
I had the same problem after I got my phone and ran the update. Not only was YouTube acting up but anything that streamed, including music. To fix it I did a factory reset and now everything works perfectly. Try doing that and see if it helps.
Same issue here for me on WiFi with 45 Mbps down Internet speeds. Came from the Atrix on gingerbread and had similar issues with the updated YouTube app. I have some buffering issues via the PC browser but nothing that sucks this much. Right now I'm on CM 10.1. I see a lot of reviews on the play store with similar complaints on other phones. Definitely gotta be an app issue.