I had the same problem with my 2006 Macbook CoreDuo. I could only join to my D-Link router if the macbook was in the same room. Even one wall could break the connection and I could not join again. I ordered a Broadcom BCM94321MC 802.11n card and after exchanging the default Atheros the problem was gone - and it's much faster than the Atheros even for g type connections too. I assume a new card would solve your problem.