Really make sure the stuttering is not from the stream being cached first.
Did you try playing a DVD or a local video? Do they play smooth or do they stutter as well?
If playing all local sources do not stutter or stall or jump around then I would fault the network.
Try all video sources on the internet.... youtube,liveleak,younow,msn Do they all stutter?
If they all stutter then also make note of the time you are testing. You might be testing at peak internet usage across the USA.
Try testing when peak usage is not at its peak. If you like staying up late try at 12 in the morning. Different times do get different results.
SInce you did say you bought a new video card.... then did you use something like a video driver scrubber?
Sometimes video driver conflict with each other.
Once we have diagnosed it is not your video card and not the internet then we can assume it is your hard drive. how may tray icons are on your task bar?
Antivirus sometimes can be blamed for choppy video.
5 years ago a client also had choppy video but her old hard drive was more than 15% fragmented. She had the pc for years and never ran defrag.
Noticeable hard drive slowdowns happen when the pc is fragmented at 2% or more.