Many people in this forum report issues that are often fixed by using the semiconductor manufacturer's reference drivers. It seems that some brands of network equipment modify the drivers slightly and this leads to additional problems with the hardware. Unfortunately the original reference drivers are often very difficult to find.
I've just stumbled accross an excellent site with reference drivers for many popular WiFi chipsets including Atheros, Broadcom, Ralink, etc.
It also includes charts with many models of wireless so you can find the chipset your card uses.
http://atheros.rapla.net/
http://broadcom.rapla.net/
http://conexant.rapla.net/
http://inprocomm.rapla.net/
http://intel.rapla.net/
http://marvell.rapla.net/
http://ralink.rapla.net/
http://realtek.rapla.net/
http://sis.rapla.net/
http://ti.rapla.net/
http://zydas.rapla.net/
Wireless reference drivers and chipset information
Linear Mode
