I just reinstalled the 8 and updated to 8.1 Update 1.
I confirmed 8 smoothly runs stream. On 8.1 Update 1, the stream stutters. Actually, I found the sound also became jitter on 8.1 Update1. The only drivers I can see in the Program & Features are Nvidia Video driver and Nvidia sound driver, which are automatically installed by the installation process.
I think the culprit is poor coordination between sound driver, Windows 8.1 and Adobe Flash. Machine has several audio ports on motherbord and also HDMI on graphic card. So I need not only Nvidia High Diginition Audio driver but also Realtek High Definition Audio driver.
After that I tried to install/uninstall Realtek's sound driver or Nvidia's sound drivers, nothing worked to fix stutters.
I'll post later if I found other things.