Pixelated jpeg in premiere pro export

[clompchmac]

In my timeline, the preview looks great. The image is very large, around 3k X 3k of the sky.

However, when I export it, it turns into weird pixelation/blurriness. Here is the video link: https://youtu.be/CWsEortyYrI

My output settings are:

Match sequence settings >unchecked

Format: H.264

Preset: Custom

Render at maximum depth > checked

Use maximum render quality > checked

What can I do? The image of the sky is slightly key framed to scale slightly back and forwards to give a sense of movement... 

Thank you!

[Ann Bens]

...so we dont have to download unknown files: we help you, you help us. Thanks! 

Videos can be posted on youtube.

[clompchmac]

hi - just added the video link: as you can see, the sky has a weird loading in effect, it goes a bit blurry, then sort of high quality, then blurry again...