I have two desktop computers a laptop and a smartphone. All of them connect perfectly to my router (WRT54GL with Tomato firmware), but randomly (once a week maybe) something odd happens:

For over an hour or so one of the desktop computers (Windows 7 Pro x64) will just refuse to connect properly to the router. Only that computer and only to that network. I can connect all the other machines to the router perfectly and I can connect properly to other networks with that machine. What I mean by "not being able to connect properly" I mean that the OS will just tell me "not able to connect to.." or that it will connect but then say that it has no Internet access or it connects but will then take minutes to load any website, even the router web interface.

I've tried to change from WPA2 to WEP and back to WPA2, I've tried different network adapters (one internal PCI card and one USB external card), removing networks from Windows and adding again... without making any difference. It works for some days but at the end, it ends happening at some time.

I just have no clue on what the hell could be going on here. I first thought it was the network adapter, then the router, then Windows...

Thanks in advance