Airport extreme connection issues

Question

Yesterday, I bought a new Comcast-compatible modem (Motorola SB5101) and an airport extreme.  These replaced an old (5 years or so) SB5101 and an equally-old airport express, which was too old for my new MacBook Pro to be able to manage it through airport utility.
I replaced both components because I got sick of my internet connection dropping constantly, many times a day for the last few years.  (I know, but it's true!)
Comcast insists that the signal coming into the house is fine and is constant, so it's not the ISP dropping the connection.
The cable comes into the house, into the modem.  Ethernet goes from the modem into a Sprint AirRave (Sprint says that must be how it's set up).  From the AirRave, the ethernet goes to the airport express.  From there, I can connect computers and devices wirelessly, but from the airport express, an ethernet also goes into my home distribution panel, so that in each room of the house you can plug directly into the ethernet (about 8 outlets).
The problem happens whether or not the AirRave is installed.
Sometimes the connection drops for a few seconds and re-connects.  Most often when it drops, and I go into Network Preferences Diagnostics, Wi-Fi = green light; same with Wi-Fi Settings, Network Settings, and ISP.  But Internet and Server = yellow.
Sometimes running the diagnostic fixes it and it re-connects.  Other times you must reset the airport and/or the modem.  Sometimes it will not re-connect even after multiple times re-setting those deivces.
Since installing the airport extreme yesterday, sometimes I get a message on my Macbook Pro that says my MAC address is being used by another device (?).
I have no knowledge about networks AT ALL, no idea how my computer can have the same MAC address as anything else, no idea what specific settings I should be using -- I always have let the setup happen automatically.
If anyone has any idea how to solve this I'd be very grateful.  I'm not sure whether I've given enough details, like I say I lknow NOTHING about how to set up a Network.
It was a nightmare setting it up yesterday, once I got it up according to instructions, I was in an endless loop -- every time I tried to open a page in my browser, I was taken to the Comcast Setup page instead, asking for my phone number and account number.  Entering this took me to a new page saying "unable to connect with our system."  I was on the phone with Comcast forever trying to fix it, resetting the modem and airport over and over again.  Eventually somehow it worked, but later that night (and again already this morning) the connection has dropped a number of times.
I thought getting new components would fix the problem, now I'm so frustrated I don't know what to do.
Anyone?
Thanks!!
-- Craig.

Answer 1

Since installing the airport extreme yesterday, sometimes I get a message on my Macbook Pro that says my MAC address is being used by another device (?).
This usually indicates that there may be something amiss with the NAT service or that each network client cannot access it properly.
To verify that the 802.11n AirPort Extreme Base Station (AEBSn), Motorola modem, or Internet service is NOT the issue, we will temporarily remove the AirRave from the network equation.
To do so, please power-down the modem, AirRave, AEBSn, and computer(s). While they are all powered-down, disconnect the Ethernet cable going to the AirRave.
Next,
Disconnect the AirPort base station from the AirRave and Ethernet distribution patch panel.
While all of the devices are powered-down, perform a "factory default" reset on the base station. This will get it back to its "out-of-the-box" configuration and make setting it up much easier, especially if you use the "Assist me" process within the AirPort Utility. (ref: Resetting an AirPort Base Station or Time Capsule)
After the base station resets, go ahead and power it back down.
Temporarily connect the Ethernet cable coming from the modem to the WAN port of the AEBSn.
Reconnect the Ethernet cables coming from the AEBSn's LAN ports to the Ethernet Distribution Patch Panel.
Power-up the modem; wait at least 10-15 minutes to allow it adequate time to initialize.
Power-up the AirPort base station; wait at least 5-10 minutes. Note: The AirPort's status light may continue to flash amber after it has intialized. That is because, there may be some additional configuration items necessary, like setting up wireless security, before the overall setup is completed to get a green status.
Power-up your computer(s).
In this basic configuration, the AirPort base station will broadcast an unsecured wireless network with a Network Name (SSID) of Apple Network NNNNNN. Network clients, connected to the base station either by wire or wireless, should now be able to access the Internet through the ISP's modem. Once Internet connectivity has been verified, you can use the AirPort Utility to configure the base station for wireless security and any other desired options. Please post back your results.