It's odd, but I often have a difficult time watching something on YouTube without constant buffering, and this is during the middle of the day when my speed is at least in the mid 20s. I'll watch the progress bar, and it'll move a little bit ahead, then stop, then move a little ahead, then stop, often never making it more than a few seconds ahead of what is presently playing, which makes zero sense. This often happens even after the resolution drops to 144p. I already have it set up to always play videos in a lower resolution, so it never starts higher than 360p.
Now, if I go to play the same video at 4am, when my speeds are practically the same, the progress bar will zip across to the end and I will have no problem.
I suspect it has to do with the fact that the high speeds during the early morning are much smoother, in that the middle variance on the speed test is low and the graph bar is fairly even, whereas the tests during the middle of the day, or even the evening when my speeds are good, have a middle variance that is high and a graph bar that is all over the place.