Browser video glitch on iMac Pro - updated Silverlight fixed it

Just got a refurbished iMac Pro and migrated my old iMac files to it.

During the migration I was prompted to update so the Pro's running OS 10.13.6

The first evening of use that started having glitches in the video when running streaming video in Safari or Firefox.

If the video was paused for a while, when I hit Play the video would skip ahead like it was scanning in fast forward, stopping, and skipping ahead again, sometimes just "fast forwarding" to the end.

Quitting the browser didn't work, but restarting the machine did, at least until the next time I paused video.

I typically run several clients of a science game in background while doing other things, and the games have a fairly high impact on the CPU and GPU, but I could run up to six game clients and stream HD video at the same time on the old iMac ( a 2015 Retina 5k IMac) so I figured something else was going on.

Removing and reinstalling the latest version of Silverlight ( version 5 at this writing) solved the issue.