Cax80 Drops Connection, But Wifi Works.

I have a cax80 wifi modem and a 1.2 GB Comcast Connection. The modem drops the connection to my pc that is connected via ethernet cable. Sometimes the connection will be fine for several hours, then it starts dropping, reconnecting, over and over and that can go on for hours. The wifi stays connected and my phone, TV's etc...work just fine. I have tried different ethernet cables, different coax cables, installed different drivers for the Realtec PCIe 2.5 GbE NIC card. Of course I've restarted, and even set the modem back to factory settings mulitple times. I bought the device used and was told it worked fine (yes, it might be a lie). My old wifi modem worked absolutely fine. I only replaced it to take advantage of the speed increase and needed a multigig modem. ISP says the signal is reaching the modem with no problem. Again, the wifi doesn't disconnect, just the connection from the modem to the pc. When it happens, the mutilgig light goes off on the modem as well. Any suggestions would be greatly appreciated...as I'm about to take a hammer to it any day now.