Strange connectivity problems?

I have an AT&T wireless router hooked up to my Comcast cable modem, and am accessing it using my Dell Inspiron 600m laptop's Centrino card. I've never had any problems until last night, and this one is very odd to me...here are the details:

1. 7:30pm, checked email and surfed Web, no problems
2. 7:45pm, closed lid (system standby) and took unit downstairs
3. 7:50pm, re-opened lid, wireless card found my router but would not send/receive data (email = "mail server not found," browser "page cannot be found")
4. Took unit back upstairs thinking I was too far from the router and it couldn't get a good signal
5. Right next to router, same problem - wireless card connects automatically to the router, but does not send traffic through modem.
6. Power cycled all 3 devices and reconnected in order cable modem ->

router -> laptop. Problem still present.

1. Called Comcast (ISP) - no issues on their end, checked all culprits on mine. Connected laptop direct to modem via ethernet cable, internet connection worked fine.
2. At the time, my router was not encrypted (lazy), so I thought maybe a hijacker was causing the problem. I went into my router and reset it, including a 128-bit WEP key.
3. Power cycled everything, wireless connection worked fine.
4. Closed lid of laptop again - 15 minutes later I came back, same problem - wireless card connects to router, but doesn't send traffic through the modem.
5. Plugged in ethernet cable direct to modem again, internet connection works fine.
6. Power cycled all 3 devices again, this time wireless card found the router and sent traffic through modem just fine.
7. Closed laptop lid again (system standby), opened it later and experienced the same problem.
8. Power cycled router only, did not fix - still connected to the router via wireless card but no traffic through modem.

So it seems to be this intermittent loss of connectivity between the wireless router and the modem, even though they are securely connected with an ethernet cable, and it seems to only happen after I fire up my system again after going to "standby" and killing the wireless connection.

The wireless card -> router connection seems to be fine, and The modem -> ISP connection seems to be fine.

Any thoughts on what this could be? At this point I'm entertaining the following notions:

Thanks for any advice.