Artifacts

I’ve noticed in several of my shows, there are artifacts that only occur when the video is played through plex. My server is running on a freenas machine and is direct playing to my pc. The artifacts are usually pixel blocks or artifacts that shouldn’t exist like for example a black mark that appears for a second. When I play the file directly, over my network the video is fine. I set every option to maximum quality. Maybe it is just a plex issue that everyone has, but it seems kinda odd. Any ideas whats happening.

If important, freenas machine specs:
Xeon E5-2670
LSI 9211-8i IT Mode
6 x 3tb WD Red drives
1 x 500gb Samsung 950 EVO for plex