I just got a brand new xbox 360.The online signal bar is very weak. The signal bars are fluctuating between 1-3 signal bars always. My internet provider is comcast. All the connection wires are brand new.
if you internet connection for your xbox360 is wireless then that is the biggest contributor to your connection issues. I too had a wireless network adaptor for my xbox and the signal varied depending on what time of day it was or if anyone else in the house was online at the same time.
If, however, you are hardwired to the internet (ethernet cable is plugged into the back of your xbox for internet access) and you are having these connection issues, then you need to call comcast and talk with them as to why your internet bandwidth is so variable.