Lag in Premiere Pro Linked to Apple OSX/Hardware?

Hi,

The Issue:

Several of my editors are experiencing a problem at their iMacs where one moment playback of their timeline will be smooth and the next it will be so laggy that they want to walk away from the project.  All of the iMac clients are in a managed environment with open directory and profile manager (so updates to both the OS and Creative Cloud have been suppressed).  I turned off our backup system (in case that was trying to complete a job in the background) and the problem persisted.  The backups usually run in the overnight but occasionally can take a bit longer to complete based on the number of clients being backed up and the size of the files being checked in with version changes etc.  The footage (AVCHD) has no effects applied.  It shows up yellow in the timeline as though rendering would improve performance but it technically shouldn't require a render just for smooth simple playback. 

The Resolution:

Interestingly enough, if the client's network cable is unplugged from the back of the computer the lag goes away.  That leads me to believe something is trying to run in the background that requires a network connection but my users don't have the ability to pull up Activity Monitor with their permissions and I can't accurately replicate their issue since I'm on a Mac Pro. 

The Specs:

• OSX 10.11.6
• iMac Retina 5K 27" Late 2015 Model
• 3.2 GHz Intel Core i5 Processor
• 16 GB 1867 MHz DDR3 Memory
• AMD Radeon R9 M380 2048 MB Graphics Card
• Premiere Pro CC 2015.4

I know there are a few updates for the Creative Cloud environment and OS that I have pending but I don't like to update in the middle of projects so since things have been pretty stable I have been waiting on those.

If you have any suggestions or ideas we can try I would certainly appreciate it.  Thanks so much!  I look forward to hearing your thoughts.