First the usual drill for Premiere Elements 8 users.
1. Are you running the program as the 8.0.1 Update?
2. Do you have the automatic Background Rendering and AutoAnalzyer features turned OFF?
3. If you have a GPU video card, do you have "enable GPU playback" selected in preferences (Edit Menu/Preferences/General)?
What are the video and audio compressions for this .mp4 file? It appears to have a 25 progressive frames per second frame rate.
What have you selected as the project preset? Is there a red line over that file on the Timeline? If so, do you render the file by pressing the Enter key of the computer main keyboard to get the best possible preview in the Edit Mode monitor playback?
Let us start here and then move forward based on your answers.
Ok, so I did the upload and this happens: https://www.youtube.com/watch?v=iFxQ-pU4YJg
The quality looks fine, but as the video goes on; the quality continually gets worse, notice towards the end the ground is a mess.
I have something for you to try.
What did you use for project preset? I am assuming PAL AVCHD Full HD1080i25 since there is no DSLR 1080p setting in version 8.0/8.0.1.
(Did you say what the recorder is offering in the way of video compression to go along with the .mp4 container format?)
Take that Timeline and export it to a file saved to the computer hard drive.
Share/Mobile Phones and Players/Apple iPd and iPhone/and preset = iPod and iPhone - High Quality.
Under the Advanced Button/Video Tab of that preset, have your Export Settings look as follows:
Then upload that file (H.264.mp4) to You Tube at the YouTube web site. What is the quality of the uploaded video?
We will be watching for the results.