Same exact problem for a couple of months. iMac running DVR and ATV 4.
Thank you for confirming that this is not an isolated problem. I watched The Orville tonight and all was fine until 47 minutes into the episode and all of a sudden it started messing up. However, all we get is DVR Users posting stuff, and none of the developers reading this one, it seems.
The time frames here range from one week to multiple months. That's not helpful in tracking things down.
This thread was created about an issue specific to Windows servers that has been resolved. In that case, the speedtest always reported a consistent speed, and only seeking was slow.
You're reporting varying speeds and also stuttering during playback, and using a Mac server. I have a similar setup and have not noticed any problems with any of my recordings.
Next time the issue occurs, go to the bottom of the Settings tab and click Submit Diagnostics.