Internet Issues in Neighborhood...how to solve?

We are having the exact same problem. We thought it was the Internet coverage so we went online and upgraded for the giga blast. Which is about $150 more. So the cable guy came he checked all of the Internet connections and he indicated that he found that there is a major interference somewhere within our home. After checking the Internet box in our closet looking at the cable wire connected to the TV, He came to the conclusion that “something”was interfering with the connection. That something was never clear. So he replaced the modem box with the new one which is the giga blast. Which I have since learned after the fact that the giga blast only has to do with the fast upload of items on the Internet, it has nothing to do with connection, it has nothing to do with the total number of devices, it has absolutely nothing to do with fixing the spotty connections throughout the home. After the worker left, we realize he half connected the new modem box and he left the old modem box. The end result, our Internet connection is worse now than what it was before. We are having major problems connecting several important devices within our home. We are from California, what we know for sure is that Cox is charging two times more than what California charges,  for the top of the line Cable and internet. I’m sorry I don’t have a resolution but we sick of Cox and the “no nothing employees”.