There are just too many issues to deal with...Busy server, congested broadband highway, your hardware, your software, etc. just too many to count. You are using the computer for this or one of those media devices? Your broadband is FiOS, cable, or DSL?
Using Firefox / Win XP. SP3
I've long given up on trying to diagnose & fix my irritating problem of hiccuping, herky-jerky playback when I want to watch online vids. I tried to ask Netflix people. Nothing. I'd hear the machine accelerate & whenever it did that, the choppy playback occurred. It would make that sound for a few seconds, then stop. When the sound stopped, the video would play smoothly.
I don't know if it's some kind of registry corruption or what.
I tried downloading some video acceleration program & adjusting certain settings in the Troubleshoot tab of "hardware acceleration". (set to full)
What now?

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