using same ethernet cable with two different xboxes (at different times) one stays connected the other constantly disconnects from xbox live

Are you using you router to assign IP or are you using a static IP? first thing I would do is check your router to make sure there is nothing else using the same IP as the 360 on your network.I would make sure you don't have an IP conflict. If that is not it I would change one of the 360's IP address.

There is no ip conflict.  Only one xbox is connected at a time.  Due to restrictions of the router, only one host can be port forwarded for xbox Live.  Therefore, to have Open NAT with both, the one has to be configured to be identified as the other  for port forwarding to work.  It doesn't make any sence why one has no connection problems but the other constantly is disconnecting from xbox LIVE while playing Halo Reach.

So just to make sure the one having issues is the one that doesn't have the alternate MAC set correct?

Try getting the update on a jump drive again manually - make sure that the core file on the drive is the $systemupdate folder - not the files inside of it, or it inside of another folder.

Once you have that downloaded again go to your System Settings> Memory section and highlight your HDD or MU.

On the controller hit the following succession as fast as you can: LB>RB>X>LB>RB>X

This will clear out old update files.

Once that is done open the device options on the storage device and clear system cache.

After all that is done restart the console with the USB drive plugged in to get a fresh install of the update done.

See if it disco's after that point, and post back the results.

I did all you suggested Antillion, but did not work.  I've unistalled the game and re-installed and again cleaned system cache.  I finally got another router connected by putting the combo router/modem verizon gave me into bridge mode.  All I could get was a strict NAT connection with the second router and now cannot get connected at all with that configuration.  I've disconnected second router and set verizon's back to default and then enabled it to get an open connection, but after recovering gamertag I can not get connected to xbox live.

Ok, what error message are you getting when you hit more info on the connection test?

Too honest, I am not sure I even understand any of your post: are you saying that you have two consoles that you connect to router using the same ethernet cord, but you only connect one at a time to the router?  I don't know why you would want to do that.  Could you explain?

Why not just set up port forwarding to both xboxes? All you should need to do is to enter the IP address for the one console and open the ports to that one, then enter the IP address for the other console and open the ports to that one, hit save and you are done. Both consoles are now port forwarded and could still be connected to the router at the same time. No need to keep plugging and unplugging each console.

Yeah...that is what I was thinking.  I used to keep two consoles pluged into one router for several years.

On the Westell 7500 there is a known issue with port forwarding - one device can get an open NAT by using the Static NAT feature - the DMZ and portforwarding do not usually work.

How to forward ports on westell 7500

You can add more than one device to the port forwarding.