Have you got a 'dial on demand' option set as a default on your router? If so, there is a possibility that the ADSL connection will drop after a period of inactivity.
Is it the W-LAN connection or ADSL internet connection which drops? If it's the ADSL, try looking into the router configuration and changing 'dial on demand' to 'always on'. But if the W-LAN drops out even with the computer right next to the router, it must be something else.