On what computer operating system is your Premiere Elements 9 running? Are you working from the 9.0.1 Update of 9?
To your immediate question...my first impression is that you are dealing with interlaced video and interlacing problems. Your source as described is
1920 x 1080 @ 25 interlaced frames per second (counterpart of 50 fields per second).
Can you set your camera to record with a setting for progressive instead of interlaced video? If so, do that.
If you upload interlaced video to YouTube, then you are leaving the deinterlacing situation to YouTube.
You could export your Timeline to a type which includes deinterlacing and then upload that export to YouTube at the YouTube web site and determine if that improves your situation.
I would expect this situation to apply to Premiere Elements (any version).
Please review, consider, and let us know the outcome as your schedule permits.
My system was PE9 Windows 32bit Xp Intel Core Duo when I last uploaded to Youtube but I have just upgraded to Windows 7 64bit Intel i7 8 core and purchased PE13.
Yes I know progressive doesn't suffer from this problem.
Shooting in progressive is not an option because I shoot everything primarily for a big screen (blue ray or DVD) and everything here in Australia is PAL interlaced like in Europe.
The max progressive is 720 on many TVs with 1020 being only interlaced.
Haven't had a chance to try PE13 on Youtube yet but I can't see anything obvious in PE to create a progressive file from my interlaced edited stuff without halving the vertical resolution. Any suggestions?
The odd thing I cant work out is why the "wriggly" vertical distortion is about 20 wriggles top to bottom whereas interlaced is far higher (1020/2=510) . Eg a telegraph pole looks like a snake with 20 wriggles.
Do you have any of these wriggles in a video now on YouTube so I could take a look at what you are experiencing?
What happens if you take your 1920 x 1080i25 Timeline content and export it specifically to a AVCHD.mp4 file
with Presets = MP4 H.264 1920 x 1080p25
and then upload that file from its hard drive save location to YouTube at the YouTube web site?
Add On...when I mention your source as 1920 x 1080i25, I have not forgotten that you are recording 50i which is 50 fields per second the counterpart of 25 interlaced frames per second.
Unfortunately the upload I had has been taken down by Youtube
because I haven't accessed it for quite some time.
I will have to try again in a few days time
If this is an interlaced matter, it will be interesting to see how that Premiere Elements 1080p25 export file plays on YouTube when the file is uploaded to YouTube at the YouTube web site.
I would still use the 1080i25 project preset with the 1080p25 suggested export.
Looking forward to your results as your schedule permits.