Wifi works for about a day. (router issue)

I have a LinkSys WRT160N and Two bbs. One is a Bold 9700 and the other a Torch.
Both will connect to the router fine and usually work for about a day or two, then i notice the BIS has switched back to the mobile network. Resetting the router (plug) resolves the issue. All pcs are ok so i think its something in the router peculiar to the BBs. It drives me a bit nuts.

Anyone resolve this ?

thanks !