Video Artefact Problem - Sony vegas Movie Studio Platinum 9

[abeelandig]

Does anyone know what causes the horizontal lines that appear when there's movement in this video?
http://www.youtube.com/watch?v=lI8fQost7w8
They are not there before uploading to Youtube, and only appear after Youtube has processed the video.
Is there any setting to prevent them occurring?
Thanks in advance.
Sony Vegas Movie Studio Platinum 9.

[Baldrick]

It's interlaced(see http://en.wikipedia.org/wiki/Interlaced_video for an example ). So try deinterlace with vegas movie studio if possible.

[manono]

They are not there before uploading to Youtube, and only appear after Youtube has processed the video.

You're claiming YouTube introduced the interlacing? Impossible. Whatever software you were using to view the video was deinterlacing it. As Baldrick suggests, deinterlace it before uploading it.

[abeelandig]

Thanks.
In Vegas' settings, I have:
Project>Properties>Video>Deinterlace Method>Blend Fields ie it's not set to 'None'.
But it still seems to have this problem.
I am new to Sony Vegas. Is this the correct way to render deinterlaced video?
As a test, I just re-uploaded an old video to Youtube that I had on my hard disk that I had previously uploaded to Youtube and know displays ok there with no interlace lines. This second time of uploading the same video now has it appearing on Youtube interlaced, suggesting that something's changed at Youtube's end, not mine.
Presumably therefore, they've changed something in the software they use to process the videos before they go live. Hope it's temporary, since it spoils the video...

[manono]

If it were the same video it would be rejected for being a duplicate. It's happened to me several times when I forgot I'd already uploaded a video. Or had you removed the old version from YouTube first, before uploading it again?

And 'Blend Fields' deinterlacing is about the worst possible way to deinterlace. But I've heard tell that all the Vegas deinterlacers suck.

[abeelandig]

I changed a couple of bytes in a hex editor to get round that so the MD5 hash differed.