airport suddenly will no longer connect to network!

I have a mysterious situation that is driving me nuts! If anyone can shed some light on this I'd be forever grateful!

My almost brand new Intel iMac has suddenly stopped being able to connect to our wireless network for internet. We are using a Linksys router and have one Windows XP and 3 Macs that have worked just fine for many months.

The iMac is in a separate building from the router and modem - but the signal is good. iStumbler shows 52% on average and no other networks in the area. My older iBook still works fine from this building. So signal strength doesn't appear to be the issue.

Then, if I lug the iMac over to the building where the router is - it works!
If I bring it back to the other building - it displays the name of the network, I select it and it asks for WEP password and all I get is the "there was an error connecting to the network" message.

None of the configurations have been modified. There is no interference that I can tell. The overall scenario is the SAME except this iMac won't connect from this particular location anymore.

The local Apple store said to bring it in and maybe the airport card is failing, but somehow, I doubt that will be the case, but who knows.

Has anyone else had experience with a similar situation? What is going on here? All the other computers continue to work fine.

Thanks!