Same issue. After updating Windows 8 to Windows 8.1 Update 1, Twitch.tv Flash streaming became stuttering. It keeps stopping 1 sec, fast forward the buffer and playing a few sec finely.
I tried IE11 and FF29. The issue is worse in IE11.
I did updating the Flash, video drivers, Windows Update. Power Option to High, Compatibility View setting, Use software rendering. Nothing worked.
Now I have to reinstall 8 and other softwares. SIGH...
It just seems Adobe doesn't give a damn about this issue and thinks its Microsoft's problem because they did nothing for 10 months... Ironically, my iPod touch 2010 plays twitch.tv smoother than Desktop PC lol. This reality is no joke.
Don't install Windows 8.1 if you are relying on Flash to watch streaming. You'll waste your time.
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windows 8.1 Update, Core i7, 8GM RAM, C: 64GB, D: 2TB, GTX460