Losing network connection from time to time

Question

I am running a Ubuntu 11.10 desktop, the system has two nic's installed.

eth0 is managed by the networkmanager, eth1 is a bridge interface

/etc/network/interfaces
# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface (managed by networkmanager)
# auto eth0
# iface eth0 inet manual

# The secondary network interface
auto eth1
iface eth1 inet manual

auto br0
iface br0 inet static
address 192.168.7.211
netmask 255.255.255.0
bridge_ports eth1
bridge_fd 0
bridge_stp no

arp-scan per interface shows a different output

arp-scan --interface=eth0 192.168.7.0/24
Interface: eth0, datalink type: EN10MB (Ethernet)
Starting arp-scan 1.8.1 with 256 hosts (http://www.nta-monitor.com/tools/arp-scan/)
192.168.7.1         00:1b:c0:34:45:12   Juniper Networks
192.168.7.254   00:14:bf:a2:79:05   Cisco-Linksys LLC

arp-scan --interface=br0 192.168.7.0/24
Interface: br0, datalink type: EN10MB (Ethernet)
Starting arp-scan 1.8.1 with 256 hosts (http://www.nta-monitor.com/tools/arp-scan/)
192.168.7.1         00:1b:c0:34:45:12   Juniper Networks
192.168.7.254   00:14:bf:a2:79:05   Cisco-Linksys LL

arp-scan --interface=eth1 192.168.7.0/24
ioctl: Cannot assign requested address
WARNING: Could not obtain IP address for interface eth1. Using 0.0.0.0 for
the source address, which is probably not what you want.
Either configure eth1 with an IP address, or manually specify the address
with the --arpspa option.
Interface: eth1, datalink type: EN10MB (Ethernet)
Starting arp-scan 1.8.1 with 256 hosts (http://www.nta-monitor.com/tools/arp-scan/)
192.168.7.1 00:1b:c0:34:45:12   Juniper Networks
192.168.7.210   f4:6d:04:af:01:d9   ASUSTek COMPUTER INC.
192.168.7.211   f4:6d:04:af:01:d9   ASUSTek COMPUTER INC.
192.168.7.254   00:14:bf:a2:79:05   Cisco-Linksys LLC

only when 0.0.0.0 is used as source address the ip adresse 210 and 211 ar shown, but they have the same mac address, but they are on different nic's.

output ifconfig
br0       Link encap:Ethernet  Hardware Adresse f4:6d:04:ae:e8:d0  
          inet Adresse:192.168.7.211  Bcast:0.0.0.0  Maske:255.255.255.0
          inet6-Adresse: fe80::f66d:4ff:feae:e8d0/64 Gültigkeitsbereich:Verbindung
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metrik:1
          RX packets:5297 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1896 errors:0 dropped:0 overruns:0 carrier:0
          Kollisionen:0 Sendewarteschlangenlänge:0 
          RX bytes:463074 (463.0 KB)  TX bytes:115018 (115.0 KB)

eth0      Link encap:Ethernet  Hardware Adresse f4:6d:04:af:01:d9  
          inet Adresse:192.168.7.210  Bcast:192.168.7.255  Maske:255.255.255.0
          inet6-Adresse: fe80::f66d:4ff:feaf:1d9/64 Gültigkeitsbereich:Verbindung
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metrik:1
          RX packets:120669 errors:0 dropped:0 overruns:0 frame:0
          TX packets:93541 errors:0 dropped:0 overruns:0 carrier:0
          Kollisionen:0 Sendewarteschlangenlänge:1000 
          RX bytes:124115299 (124.1 MB)  TX bytes:8299417 (8.2 MB)
          Interrupt:16 Basisadresse:0x8000 

eth1      Link encap:Ethernet  Hardware Adresse f4:6d:04:ae:e8:d0  
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metrik:1
          RX packets:6769 errors:0 dropped:0 overruns:0 frame:0
          TX packets:3373 errors:0 dropped:0 overruns:0 carrier:0
          Kollisionen:0 Sendewarteschlangenlänge:1000 
          RX bytes:1006597 (1.0 MB)  TX bytes:244116 (244.1 KB)
          Interrupt:18 Speicher:fa700000-fa720000 

lo        Link encap:Lokale Schleife  
          inet Adresse:127.0.0.1  Maske:255.0.0.0
          inet6-Adresse: ::1/128 Gültigkeitsbereich:Maschine
          UP LOOPBACK RUNNING  MTU:16436  Metrik:1
          RX packets:2288 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2288 errors:0 dropped:0 overruns:0 carrier:0
          Kollisionen:0 Sendewarteschlangenlänge:0 
          RX bytes:288354 (288.3 KB)  TX bytes:288354 (288.3 KB) 

as a result my network connection is lost from time to time.