Eliminating a signal issue starts with a tech visit that identifies the source of the signal issue. If it was found to be on your side of the demarc, the tech could have repaired it at your expense. If it was from the tap to the demarc, the tech should have repaired or replaced it. If the issue was on the main line side of the tap, the tech should have escalated the issue for a line tech visit to repair the signal issue. If techs have already identified a main line signal issue, they should have escalated the issue. Your best course of action now would be to contact cox.help@cox.com. Include a full description of what's been done to date along with your full name, service address and a link to this forum discussion.
Edit: LaToyaW posted while I was typing. So, do what she suggested.