Only 1 Up Channel Bonded & 51-55 Upstream / Bad Wiring or Buy Modem?

Modem: Netgear C3700-100NAS

Connection: No spitters in the house.  No TV. Modem combo is connected directly to the coax coming from the wall. Coax from wall is closest to the outside connection. 

Problem:

I'm having some ongoing issues with my modem requiring frequent resets.  I believe these problems to be related to my 51-55 dBmv and my only one locked bonded upstream channel.  I believe this is a line problem, and not a modem problem

Attempted Solution:

Cox tech acknowledged my 51-55 dBmv .  The tech suggested that i buy a new modem because mine was more than 3 years old.  I will post excerpts of this chat log below. The tech offered to send a rented one to me. However, I discovered he modem came with a $20 self installation fee, so I refused this option.  If the modem was the problem, i would rather use that $20 to buy a Netgear CM600 or an Arris sb6183 and mate it to an Archer C or A7.  

Question:

Does the tech's suggestion to buy a modem sound right to you?