WRT55AG- Wired Connectivity Issues

Question

I have a BEFW11S4 that has worked great for at least 2 years now.  My setup is cable internet (from Comcast) to the router, with a wired connection to a desktop and a wireless connection to, at most, 2 laptops and an adapter I use to feed ehternet to my xbox 360.
A few months ago I purchased a WRT55AG, version 2.  With the initial firmware, I was able to replace the BEFW11S4 fairly easily, and seemed to be ok for a day or two.  Then I started having issues with internet connectivity (It was a while ago, but I believe the wireless network stayed up while internet was intermittent and/or disappeared completely to both wireless and wired devices.  I can't recall if the desktop was have local area connection issues, or just internet dropped.).  I put the BEFW11S4 back in and haven't had time to start testing again until recently.
Since I've tried to set up the WRT55AG again, I haven't been able to get an internet signal from my modem to the router.  Further, when I have my setup modem-->WRT55AG-->wired desktop, I get an error saying that my local area connection is down.  Modem-->BEFW11S4-->desktop gives me no such error.  Wireless network connectivity works (although it seems intermittent), so I have been able to wirelessly apply firmware upgraded v1.79.  I have tried every reset sequence possible between modem, router, and desktop.  I am not a network wiz by any stretch of the imagination- I tried to feel my way through some of the MAC address cloning using the wireless connection to the router, but I'm not sure that would help if I can't get a wired connection.
If I can't get the router to acknowlege a hard line coming in with internet, or my desktop to acknowledge the wire coming in from the router, I'm not sure what I can do.  Any advice?

Answer 1

Some more information- when I connect the WRT55AG to my Comcast (Motorola SB5100) modem, the 'Internet' light on the router does not respond.  The same when I connect via hardline to my functioning Linksys wireless-b router.
However when I connect via hardline it to my access point downstairs (a wireless Motorola adapter with an ethernet output), the internet light comes on solid for a few seconds and then blinks.
By the way, with everything I try, when I plug the router in, Power is a solid green and Wireless A and Wireless G lights blink green.  DMZ light is off, as are the port and internet indicators.
Any help would be appreciated.  Wireless seems to work fine but I can't get an internet signal in through the hardline.