Constantly dropping internet connection / abysmally slow download speeds

I recently shifted from a Cox DOCSIS 3.0 modem for internet / telephone service to the Panoramic WiFi modem/router combo and have had nothing but problems since.  I have swapped out the first Panoramic modem for a second, thinking it was an equipment issue.  The problem(s) manifest itself in multiple ways 

1.  When plugged directly to the modem via Ethernet, the connection keeps dropping for 2-10 seconds.  I watch the icon switch from wired connection to wifi and back again constantly. 

2.  Internet speed performance is abysmal.  Ethernet download speeds at 12-15 mbps, despite being on the mid-tier plan promising download speeds of 100 mbps.  Surprisingly WiFi speeds are better.

3.  Hardwired NAS keeps disappearing from the network and reconnecting intermitently

4.  Sonos speaker set-up keeps having to be reset and reconnected (combo hardwired bridge with wireless speakers).

Problem is consistent across two different laptops (Windows 10), iPhone and iPads.  The laptops have run both Kaspersky and Webroot security/firewall software.  I have worked with Cox online & telephone support, and had a tech come out--his speed tests showed 120+ mbps.  Multiple modem resets have done nothing.  I've modified firewall settings to no avail.  I have even resorted to swapping out my ethernet cables in the event there is a bad connection.

I am hoping to get some suggestions.  The constantly dropping/resetting internet connection is maddening enough...and then there is the issue of paying for, but not receiving the promised speed.  My next step is to ditch the Panoramic WiFi and buy my own modem.  Thoughts or suggestions???