Radio problems with 505a/RFMD2958

I have an Atmel card (see details below) and I'm trying to make it work 
under Linux. With the latest CVS driver, I have the following behaviour: 
from time to time (say, once every three), when I boot the PC, I have this:

at76c503.c: wlan0: tx timeout
usb_control/bulk_msg: timeout
at76c503.c: wlan0: get_cmd_status failed: -110
...
at76c503.c: wlan0: set_card_command(CMD_RADIO) failed: -110
...
at76c503.c: wlan0: set_mib (preamble) failed:  -110
usb_control/bulk_msg: timeout
at76c503.c: wlan0: kevent: start_scan failed with -110

the card, naturally, can't find the AP, though I can see the MAC address 
for the card itself. If I rmmod the drivers and I insmod them again, it 
fails immediately within the firmware download sequence. I'm not able to 
make it work again, not even by warm rebooting the board. The only way 
is to do a cold boot.
The same card works fine under WinXP with the original drivers. The 
details are as follows:

USB : VIA Technologies (rev 1a)
Card: Atmel (ID 0x3eb:7614)
Drivers: at76_usbdfu.o, at76c503.o, at76c505a-rfmd2958.o

I welcome any comments or hints you can give me to solve this problem...

Leandro Fanzone