Horizontal "scan lines" thorugh footage I downloaded ...

Hi everyone.
I just downloaded some footage and I am getting horizontal lines through frames.
They look like bad scan lines or something?
Any ideas what this might be?
I am located in the United States and my Windows 10 supports H.264 codec so there
shouldn't be any problems.

Could be your video card or driver. Is it a 4k video?
It's kind of hard to troubleshoot it since there are so many variables.

A buyer who bought one of my files also stated that there was a problem with the horizontal lines, and requested the source file. I told him he bought HD, the source is 4K, so he needs to pay extra or cancel the deal with P5. He never replied to me.
I still don't know what is a problem with the horizontal lines.

@Wideweb, so it could be the downsizing done here on the site?
That is pretty scary.

Maybe so, or, maybe not. We need to see a file.

Could it be monitor refresh rate? I've seen scary looking horizontal banding when I play 4K high detail/high motion stuff ... I play it smaller window, or HD, and/or on different monitor it goes away.

I'd be really shocked to hear it was due to bad downsampling. That's scary!

@stevefusci -

Can you upload a frame grab with the horizontal lines for us to see?