I can't help but think there is still an issue with your cable, either at the pole or at the drop in your house. I would ask Charter to come back out and do signal tests at the pole, at the entrance to your home, and then at the connection to your cable modem (speaking of the modem, verify it is a DOCSIS 3.0 modem).

If your signal strength measurements are all within tolerances at all of those points along the chain, and you are still having issues with a new modem, then I would say it is time to argue with the ISP.

But, I am willing to bet it is a local infrastructure or equipment issue. I have had incredibly reliable and consistent performance with Charter here in the STL area.