I have 3 Deco P9 units: one at each end of my house, and one in my garage. 

Router --- Main Deco unit --- house remote unit

                          |

                          --------------- unit in garage

When I check the Deco app, the remote unit in the house is connected to the main Deco by wireless (medium strength) and PLC.  The unit is the garage is connected via wireless (poor signal strength) and PLC.  The main Deco unit is connected to a router.

After rebooting, the Deco units connect to the internet consistently and provide a steady connection.  However, after a few days any devices connected to the mesh network start to experience brief interruptions in internet connectivity, which gets progressively worse over time.  

- initially disconnections appear to be less than a second - just enough to impact on applications that are sensitive to connection issues.

- over tie the interruptions get more frequent and last longer.

I have tried switching one of the remotes with the main unit.  I still get the same behavior.

The LED's on all of the Deco units are steady white when I am experiencing the issues.  Rebooting appears to resolve the issue, but only for a few days.

All connections to the Deco units are via wifi.  I do not have any hard-wired connections to the Deco units.

I have connected a computer directly to the router and have been running Net Uptime Monitor with a polling frequency of 2 seconds.  There has been no issue with my internet service.

Settings:

Fast Roaming: Disabled

Beamforming: Disabled

Operation Mode: Access Point