This "feature" is called Dash (Dynamic Adaptive Streaming over HTTP) Playback. YouTube Center can disable Dash Playback. (Download it here) The only real caveat is that "that by disabling Dash Playback will result in 480p and 1080p not being available. This is because of a recent YouTube change." You'll only have access to 240p, 360p, and 720p. If a video is uploaded in 480p, only 240p and 360p will be available, etc.