network-manager roaming mode not working

Binary package hint: network-manager

Hi; my laptop is an ASUS A6j, my Wireless card is an Intel PRO/Wireless 3945ABG.
I've upgraded from Ubuntu 6.10 to 7.04.

In 6.10 everything about networking was ok: my wireless worked very well and the same did wired networking.
After the upgrade I found the network-manager set itself up in manual mode.
The Gnome sessions entry was also been set to "nm-applet -sm-disable".

It worked fine until I switched it (by using nm-applet) to "Roaming mode", both for wireless and wired interfaces.
After that I went in a hell, answering to nm-applet questioning me about my wireless network password.
Every time I provided the password the network-manager found itself not capable of connecting to the wireless network (password was ok...).
It found the right wireless networks, but it wasn't capable of properly managing them.
So I returned to Manual Configuration, but sometimes it simply doesn't work and I can't figure out why.
Problems occur more often at startup, or when I change from static IP to DHCP.

The other strange thing is that, in such moments of malfunction, protocols like Skype work fine,
just the browser and synaptic seem to stop working (yeah, I thinked about DNS, but they only fixed the problem sometimes, not always).

For now the working configuration is:
1. nm-applet started as simply "nm-applet" (no "-sm-disable")
2. nm-applet set to Manual configuration (no "roaming mode", neither for wired or for wireless interface)
3. DHCP for the wireless
4. static IP for the wired interface
5. DNSs provided by my ADSL modem router

My network counts this components: 3 Laptops, 1 Modem-router ADSL, 1 Access-Point Wireless (connected to the router), DHCP (on the router), DNSs provided by the router.

output of ifconfig -a:
---------------------------------------------------------------------------------------------
eth0 Link encap:Ethernet HWaddr 00:17:31:12:0C:17
          UP BROADCAST MULTICAST MTU:1500 Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)
          Interrupt:16 Base address:0xa000

eth1 Link encap:Ethernet HWaddr 00:13:02:0F:BB:5E
          inet addr:192.168.1.102 Bcast:192.168.1.255 Mask:255.255.255.0
          inet6 addr: fe80::213:2ff:fe0f:bb5e/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
          RX packets:5301 errors:25 dropped:53 overruns:0 frame:0
          TX packets:5017 errors:0 dropped:0 overruns:0 carrier:43
          collisions:0 txqueuelen:1000
          RX bytes:16639481 (15.8 MiB) TX bytes:1943597 (1.8 MiB)
          Interrupt:18 Base address:0x4000 Memory:fe1ff000-fe1fffff

lo Link encap:Local Loopback
          inet addr:127.0.0.1 Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING MTU:16436 Metric:1
          RX packets:58617 errors:0 dropped:0 overruns:0 frame:0
          TX packets:58617 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:2652330 (2.5 MiB) TX bytes:2652330 (2.5 MiB)
---------------------------------------------------------------------------------------------

contents of /etc/network/interfaces:
---------------------------------------------------------------------------------------------
auto lo
iface lo inet loopback

auto eth2
iface eth2 inet dhcp

auto ath0
iface ath0 inet dhcp

auto wlan0
iface wlan0 inet dhcp

iface eth0 inet static
address 192.168.1.242
netmask 255.255.255.0
gateway 192.168.1.1

auto eth0

iface eth1 inet dhcp
address 192.168.1.102
netmask 255.255.255.0
gateway 192.168.1.1
wireless-essid [omissis]
wireless-key [omissis]

auto eth1
---------------------------------------------------------------------------------------------

Thanks for your help guys.