Flash videos stuttering/ momentary freezing seems to be an ongoing problem with Windows, Adobe Flash player, and various browsers.
Here are some of the things I've found that seems to cause/help fix this:
1) DPC latency: This seems to be mostly caused by poorly engineered drivers (in all my Dell systems disabling the WiFi driver helps a lot)
2) OS Power Options: apparently almost any power saving option can have an effect on video playback. Changing all options to max performance can help.
3) Browser settings. Firefox is really bad about this. There are more than a few settings that seem to affect this, but I've never found one source that really tells how to optimize the browser. Your guess is as good as mine on what to do in IE (though I have found that the Metro version works better, apparently because it's a 64bit program).
Hopefully someone has more or better solutions and will post them here.