!!! UPDATE !!

Extensive testing over the last three days has revealed that the problems with playback of many YT vids, including this one, are player-dependent. The tests were done on three different fast Windows 7 Pro systems (two with nVidia and one with Intel graphics), an iPad2, and a MacBook Pro 15" Late 2011. Also rendered the video to Blu-ray format at 25 Mbs (tried both 24p and 60i) and played back on Samsung 1080p big-screen TV.

The problems, when present, affect both the GH3 and CX550V portions of the video. This tells me that there is no inherent failure of the GH3 (whew).

Results:
* Firefox 19.02 exhibits extreme amounts of tearing, and also stutter
* . Chrome 25.0.1364.172, not nearly as much tearing, but still stuttering, just not as bad as Firefox. Marginally better in HTML5 mode
* iPad 2 using YouTube app, very smooth, zero tearing, only very occasional stutter
* VLC 2.0.5 Two Flower using network stream feature to play the video shows zero tearing, zero stutter
* Blu-ray playback on TV is perfect - also fabulously sharp on other GH3 source material)

So ..What the heck is going on? Does anyone have an explanation? I'm baffled... all suggestions welcome!

---- shooting and rendering configuration ----
GH3 was using the 35-100mm F.28 lens, O.I.S. off, manual focus. I just set both cams to full auto; was not worried about white balance or exposure.

This video was rendered using Sony Vegas 11. Used the Sony AVC/MVC H.264 output format (this is the one Vegas suggests is best for "highest quality 16:9 suitable for uploading to Internet Sites". The specific customization of the default template:
Audio: 128 Kbps, 48,000 Hz, 32 Bit, Stereo, AAC
Video: 29.970 fps, 1920x1080 Progressive, YUV, 10 Mbps
Pixel Aspect Ratio: 1.000

The Vegas project settings were HD 1080-60i (1920x1080, 29.970 fps) , Upper field first, Gaussian Motion Blur, Deinterlace method: Interpolate fields.

The resultant .mp4, when viewed on my computer with either VLC or Media Player Classic HC, is silky-smooth without a trace of tearing, stutter or jitter. The pendulum looks quite blurred at the fast point in the swing, but the blur is smooth and pleasing.

A 24 Mbps version of this test is available on YT here: youtube.com/watch?v=It15gJgiKss

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