Wpi keep disconnecting

[Alberto Rizzi]

I have a Thinkpad T61 (UZ25FIT) dual boot with Xp and FreeBSD and has an 
Intel 3945ABG on board.

#uname -a
FreeBSD yoda.endor 7.0-STABLE FreeBSD 7.0-STABLE #0: Fri Aug 8 13:51:42 
CEST 2008 root at yoda.endor:/usr/obj/usr/src/sys/YODA  i386

I can load the wpi driver and configure the interface, then I can 
connect to my wifi router (linksys wag54g 1v) with WPA and surf internet 
but after a while the wpi disconnects. I think it is volume related and 
not time related, maybe a counter overflow or something other.

If I mount a NFS directory and start copying files from the server to 
the notebook, after a minute the wpi disconnects.
In /var/log/messages I can read
wpi0: link state changed to DOWN

and in wpa_cli I can read
<3>Michael MIC failure detected
<3>Michael MIC failure detected
<3>TKIP countermeasures started
<2>CTRL-EVENT-DISCONNECTED - Disconnect event - remove keys

after a minute I read in /var/log/messages
wpi0: scan timeout

and in wpa_cli
<2>WPA: TKIP countermeasures stopped

with netstat -ibh I see I moved 12MBytes

If I write "reassociate" in wpa_cli, after some seconds in var log 
messages I read
wpi0: link state changed to UP

and in wpa_cli
<2>Trying to associate .....
<2>Associated with .....
<2>WPA: Key negotiation completed .....
<2>CTRL-EVENT-CONNECTED .....

If I restart the copying process I can see
<3>Michael MIC failure detected
but wpi doesn't disconnect.

After a while wpi disconnects and I have the same messages in 
/var/log/messages and wpa_cli

There isn't a fixed amount of bytes I can transfer between two 
disconnection, sometime 10MB, sometime 100MB, but the maximum was 140MB

Please help me, I can give further info.
I have FreeBSD 7 on this PC since its release and I always had problems