does the xbox recognize the signal as being ethernet and not wireless. Check in the settings on xbox live for internet connection. Are you using the free router that comcast gives. That is a very low end router hence its free. A gigabit ethernet router is much better and your xbox should have no problem wired.
At the time of the problem on the Xbox, does the cable modem log show any errors or problems? Does the modem reset itself for no reason? Also check the router log (if one exists) for errors at the time of the problem.
I have an xbox and a couple other devices behind my comcast connection. Everything has been working fine until today. Now when I fire up my xbox all link between my gateway <> the default router handed out by comcast has packet loss. I have been in networking for over a decade now and this is the weirdest things I have seen. EVERY TIME I power my xbox on and it starts communicating with xbox live the comcast connections turns horrible.
I turn off my xbox and things start to work fine.. I
Check the ethernet connection between the Xbox and the router to make sure it is seating firmly and securely. Unplug it and plug it back in several times to ensure a good seat. Try plugging the ethernet cable into another LAN port on the router.
Cables go bad, LAN ports go bacd, heck, even the Xbox ethernet port can go bad, so process of elimination is what you need to do to isolate the problem. Going to another cable will eliminate the cable, using another port eliminates the port, and if the same problem happens, then it's either the Xbox or the router. But since everything works when the Xbox is off, then it's more likely the Xbox.
So I have been having this problem where I can connect to Xbox live and start to play in a game with four bars and an awesome connection, then at varying times after connecting to a match sometimes 10mins, other times up to an hour later I will suddenly get a symbol showing an internet cable being disconnecting. I am then told that there is a problem with Black Ops servers and find that I have been signed off xbox live. After multiple attempts to reconnect to Xbox live and then to connect to a game I am able to start playing again. Same as before, great connection 4 bars and then the process starts over again with the disconnect symbol. I tried connecting directly to my COMCAST modem to rule out the router and then same thing happened.
Decided to go get a new modem to see if that was the issue, so i bought...
I have brand new Doscis 3.0 MOTOROLA SB 6121 modem and problem still persists.
What are the modem signal levels from http://192.168.100.1 ? Assuming that looks good, run a speed test from http://speedtest.comcast.net and post the results. If that also looks good, then I'd point the finger at the Xbox or the Xbox Live service.
So the above text ^^^^ is copy and pasted and shows a possible remedy .... I have tried everything, called everyone and get the run around, i have never had this issue and have had the 20mbit Blast connection for 1.5 yrs at this address. I am having a technician come out tomorrow (wed 09/14/11). I will post after they leave with more info, HOPEFULLY A SOLUTION!!!
ISP (comcast) is taking ownership, Technician came out and did what the first technician should have done. He actually tested the lines, removed a pointless outdated splitter (15 years old) test signal and verified intermittent errors both inside and at the TAP, by him confirming these errors he scheduled a "LINE TECHNICIAN" to come adjust the lines. Received a $20 credit on my bill for the issue. He also called the line technician in front of me on speaker phone and the Technician confirmed the NODE this error is on he has been trying to find it for 2 weeks which is exactly how long i told my technician how long this problem has persisted.
So long story short I can take back the new router i dont need, also will have the lines adjusted within 72 hrs. If this doesn't clear it up in 72 hrs i will report back.