Hello,

after dist-upgrade from 11.10 to 12.04 (64bit), my wifi connection got very unstable, and keeps bugging me. Until the dist-upgrade, everything worked fine. My hardware is a ThinkPad T420 with a Intel Ultimate-N 630 wireless network controller.

In short words, the connection stops working at some random point, and I didn't figure out why yet. Unfortunately no log message seems to be printed to any log file at the time of lost connection. NetworkManager keeps telling me that the connection is alive, but neither ping to the router/switch nor DNS-lookup work anymore. Ifconfig still lists the IP-address, and iwconfig doesn't show anything suspicious either.

Nobody else in the same Wifi-network has similar problems - even though I've to admit that nobody else uses Ubuntu here.

I already searched the forum. Some people seem to discover similar issues after upgrade to Ubuntu 12.04, though I'm not sure whether they're the same - at least it's on different hardware:
http://ubuntuforums.org/showthread.php?t=1975442
http://ubuntuforums.org/showthread.php?t=1958059

Unfortunately I don't have any idea about how to futher debug this problem. As one can imagine, it's very annoying. After all it randomly breaks my network/internet connection. Any help is much appreciates

Regards,
jonas

Below follows some information about WLAN and hardware. No differences between working and non-working WLAN-connection.

Code:
$ lspci -v -s 03:00.0
03:00.0 Network controller: Intel Corporation Centrino Ultimate-N 6300 (rev 35)
    Subsystem: Intel Corporation Centrino Ultimate-N 6300 3x3 AGN
    Flags: bus master, fast devsel, latency 0, IRQ 46
    Memory at f2500000 (64-bit, non-prefetchable) [size=8K]
    Capabilities: <access denied>
    Kernel driver in use: iwlwifi
    Kernel modules: iwlwifi
Code:
$ ifconfig wlan0
wlan0     Link encap:Ethernet  HWaddr **:**:**:**:**:**
          inet addr:192.168.0.103  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: ****::****:****:****:****/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:74576 errors:0 dropped:0 overruns:0 frame:0
          TX packets:60632 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:65942630 (65.9 MB)  TX bytes:8546658 (8.5 MB)
Code:
$ iwconfig wlan0
wlan0     IEEE 802.11abgn  ESSID:"**********" 
          Mode:Managed  Frequency:2.437 GHz  Access Point: **:**:**:**:**:**
          Bit Rate=57.8 Mb/s   Tx-Power=15 dBm
          Retry  long limit:7   RTS thr:off   Fragment thr:off
          Power Management:off
          Link Quality=47/70  Signal level=-63 dBm
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:9248  Invalid misc:57   Missed beacon:0