is my modem defective??

Short Answer: Bring back the DPC3825 and get yourself a seperate modem/router. Gateways are bad. 

Long answer: Cox doesn't have any documentation or emulator for the modified firmware they load into the wireless gateways, so I can't give specific instructions but it sounds like a IP conflict. Those can be fixed with DHCP reservation. Assuming your GUI looks like the one from the manual, see page 31-33 for adding DHCP reservations. 

Thanks for your reply and link Trikein!

Will look into getting a separate modem and router. I bought this unit thru Cox, hopefully they'll take it back.  the GUI referenced does look the same, so will look thru those pages first. 

it'll run just fine online when connected but randomly the browser will display that "not connected to network" blank page, and i'll lose the connection to the modem. once manually rebooted everything's fine again. anyway, thanks again for your input. very much appreciated.

Is it happening while your using it or after a long period of not using it?

What I think is happening is something is disconnecting the wireless from the router side, but it stays connected on the computer side. Think of it like calling a friend and then having them disconnect the call but you leave the phone off the hook, eventually it will go into whats called PLO, and you get that annoying beeping sound.  With a router, you don't get a beeping sound, you just get the error "not connected to network". Part of this is releasing the IP from it's lease but since the computer never got the message the wireless was disconnected, it keeps the same IP, so when it tries to renew that IP, it can't because the router already gave it to something else, thus the IP error. There are lots of different ways this can occur with a router. Could be each time it changes channels, or the connection rate, etc. Either way, a DHCP reservation assigns the IP to the MAC address of your computer. With it assigned, there is no lease, so even if the wireless does disconnect, the IP will work just the same when you reconnect.

Let me know if you need help getting the DHCP reservation to work. You might also look under wireless settings for anything called WMM and disable it. 

thanks again for your reply!

So far, even after sitting for a while, the pc wakes and has no issues connecting. It happens only while I'm using it. the only way I'll know is because, while i'm browsing, a page will suddenly not load, then after a bit that error pops up. then if I try to connect manually windows say cant connect and will prompt to run it's "troubleshooter", which then spit out the "invalid IP..." message.

I located that WMM setting, and disabled it. it was enabled.  will look around at the other settings/etc. will most likely need your assistance with that DHCP stuff! thanks again.

also, what are good/acceptable power levels? my channels show between 2.8 -5.6. and the SNR shows generally all 37-38ish.

When the wireless connection start to experience problems do you have other wireless devices you can try testing to see if they are all impacted?

ok, this is really getting frustrating.  I am constantly getting a "you are not connected to a network..." message in my browser. page will not load and will be followed by a windows pop up warning about an invalid IP, "another computer on this network I using the same ip as this computer....".  It happens with wireless, AND my hardwired device (smart tv). When this happens, I will have to reboot the modem (cisco3825).  once in a while though, the connection will "refresh" itself and work without a reboot.  I cannot duplicate the problem, and it happens any time. today it happened 3 times already.  Since it appears it is affecting both wireless and hardwired, is this a configuration issue ans opposed to defective hardware??  I'm happy with the speed when connected as its faster than my dsl, but my dsl has NO issues like this.

Thing is, there is NO other computer on. there is only this desktop (wireless) and a smart tv hardwired to the modem itself.  if I keep "refreshing" the browser/page, sometimes it'll come back online, and sometimes ill have to reboot the modem.  and since the connection from the pc is dropped, I can't even log in to the modem.  I don't understand why the modem is reassigning an ip when there are no other devices accesing my network.

Hi Tiffany,

thanks again for you reply! no, no cell phones connected either.  I forgot to mention I do have a PS3 wired to the modem in addition to the smart tv. only the pc is wireless. also, when the "drop off" happens, sometimes the wired smart tv loses connetion too. but what's also strange is that sometimes the when the pc has that "another computer is using the same ip" message, a few seconds later, the connection is back.  I will try and factory reset the modem and see if that helps. thanks for the suggestion. I haven't tried that.