Why is playback choppy for certain clips even though I have a high speed connection?
If your line speed is being detected correctly (see next question), you may have BlackIce Defender installed, which for at least two people was causing big performance problems for streaming video. I am not aware of a resolution for this other than removing BlackIce, but the newest version of BlackIce seems to fix this, so updating to a newer version should be the first option tried. Interestingly, the buggy old SoundBlaster Live reportedly can also cause choppy playback.