Used to have the same problem with Comcast. Every single night at 9:00 my ping would jump from about 15 to 900+. Check out the dslreports forums and Charter forums if they have it to see if anyone else in your area is complaining.
Next weekend when your ping is high, do a traceroute to a common website...google, msn, whatever. reply back with the results...it could be just a high latency hop. if all else fails, or you may even want to start with this, call Charter and see if anyone else is having problems...schedule a tech to come out and take a look.