Signal randomly lost and is not consistent

I have a COX supported Motorola MB 7621 modem and ASUS RT-AC86U wireless router. Both are new and recent models. From the street the line comes into the house then about 4 feet in there is a butt connector and I have a Monoprice - 50ft RG6 (18AWG) 75Ohm, Quad Shield, CL2 Coaxial Cable with F Type Connector. This cable goes directly in the WAN connection on the modem. The LAN connection is to the wireless router. From the router I have two TVs and two printers connected via ethernet.

For the past month my connection has been having trouble of losing its connection and this lasts all day long. I understand that the coaxial cable is shared and with the increased volume the connection could be adversely effected by lowing down. But I never thought that it would be cut off completely throughout the day. Subsequently after a lot of phone support and two service visits the line does not seem to have improved. Today, I actually followed the cable form the wall and pulled it out form my wall to wall carpet. I located this connector and tightened it. Of course while he was here he was able to test the line and it was improved after the connection was tightened.

I was able to replace the coaxial cable with a new one and I thought it was working but later the same evening I was getting the same issues I will keep monitoring it in case it was an anomaly. If this continues even after all of this work is the next step to have an entirely new line run form the street to my into my house directly to the modem? I do not know if coaxial cable have a life span but the cable that is currently here must be about 20 years old. Can I have COX do this? Is it their responsibility or will it cost me?