Netgear C3000-100NAS constantly resets

1. I had to factory reset my Netgear C3000 modem
2. After this, modem randomly resets about every 15 minutes.
3. Called Cox about 4 times
4. Cox guy comes out, says there's nothing wrong with the line, "fixes" the problem
5. 30 minutes later, internet goes out
6. Call cox back, say that I need to use their modem.
7. Find out firmware needed to be updated, Cox pushes that through
8. Problem still occurring: internet randomly dies and resets the modem.

Any suggestions?

I feel like Cox doesn't really want to look into the problem because I have my own modem. They are trying to push me into renting their modem

I find it suspicious that the problem did not happen until I reset the modem. There has to be some setting somewhere that I need to select.

Any help is appreciated.