This is driving me crazy trying to figure it out. It's connected via WiFi as it always has been. But this started happening suddenly around a week ago. My YouTube has been rendered useless and it can no longer play through any video, regardless of video length.
Same for casting from Chrome or any other casting app.
Also the controller refuses to reconnect after going to sleep. I have to pull the Shield's power cord and restart to use it again.
Its only Youtube that has the buffering issue or all the apps?
If its all the apps, maybe a new router in your area is causing wifi interference. You can change your router channel to another one to avoid wifi interference.