WRT400N configured but clients can't get out?

Question

I'm a longtime Linksys user who just replaced an old WRT54G with a WRT400N (alongside a D-Link 628).  I ran through all the usual steps by connecting to the 400N with my main computer, updating its firmware to the latest, keeping most everything default to start, turned off wireless for now, and set a new password.  Didn't go nuts setting up esoteric policies, static IPs, etc.  Just set it to handle DHCP and hooked it up.
My ISP does not require any PPOE or other and you simply plug into their modem and it gives the router what it needs (works flawlessly with all past Linksys and the new D-Link when I use that).  In fact, all clients on my network (three desktops, two laptops) all get the necessary IP info from the 400N including their own IP, DNS info, etc (all checked with ipconfig /all) and they all have the correct info.
Unfortunately, the clients can't get out.  They try but the router will not pass through anything to the Internet.  No ping, tracert, ftp, browsers fail, etc.  Have reset to factory defaults and tried three or four times now and nothing.  Swapped back in the WRT54G and D-Link (both set to factory defaults, wireless off, and simply hooked into the modem) and both allow immediate access to the Internet for all clients.
All three routers pass the same information to the clients (checked with ipconfig /all) so all three modems are configured and set up the same and, according to me,  correctly.  Only the 400N won't connect to anything.
Since both the WRT54G and the D-Link immediately allow client access to the Internet but the 400N won't, there must be an odd setting I am missing but I can't see it at all.  I have gone through the setup 4 times, following the instructions to the letter (it's not that difficult as Linksys routers configuration pages are so simple to use) but each time results in clients receiving their information correctly yet not being able to get out.
Can anyone point me to the right settings to check?  I've looked through the FAQs and tried a bunch of suggestions but so far nothing.
Thanks for the help,
Robert

Answer 1

Oh yeah, been there and done that.  I have gone so far as to completely shut down the router, plug in each of my other routers (D-Link DIR-628 and Linksys WRT54G) and powered them all up from scratch (cycled power to the cable modem too and started everything up in sequence)  and confirmed they get all the same information from the modem including DNS, gateway, etc. and that the same information is passed to the clients.  In each case I wrote down the numbers to compare and they were all the same including what was reported on the WRT400N!
So, after several days of experimentation, power cycling, hooking up, etc. the WRT400N magically let me access the internet.  I guarantee you I did not change any settings from default on any of the routers (other than turning the radios off) and just merely plugged them in and started everything up!
This is very unusual.  It took about 20 restarts and tests before I could access the internet.  During that time I was able to connect to any of my internal clients (media player, spare computer, NAS, etc.) as they properly got IPs but no one could get out until a night ago when it suddenly worked.
I can't explain it.  This is frustrating since there's no obvious change (all default settings, mind you).  Of course, NOW that it gets out, I went through and set up all my usual changes such as MAC address filtering, access policy for the daughter's computer, and fixed IPs for the non-changing peripherals and it all seems to work correctly (save for one other problem I'll post later.)
 So this is "solved" but for no apparent reason.
Thanks for the suggestion,
Robert