Lots of people are asking this question but there does not seem to be a good answer. We have noticed that on our 12 Mbps connection play back stalls out all the time on HD (720p) videos now, making viewing unusable. Stalling even occurs on HQ (480p) at times. Does not matter what device we use to view – its the same everywhere. But we can play Vimeo or Netflix HD videos just fine.
One common culprit is thought to be the Adobe Flash Player, but that does not explain our Roku (with the secret Youtube channel that is no longer available). And the problem exists in different browsers on different computers. And lots of other people are experiencing similar problems, not just us. (See below – this has nothing to do with the Flash Player.)
Truthfully, Youtube is nearly unusable at this point.
The problem occurs every evening from 6pm to midnight local time, for sure. In the early morning, Youtube works okay. This means that either Youtube itself has insufficient bandwidth or the local ISP (CenturyLink telephone company DSL) is doing something to limit access to Youtube. Again, Vimeo and Netflix work fine on HD videos – the problem is only with Youtube.