bridge - lost internet connection for VM's

People, help me please.
I'm using host Proxmox 3.1 with 3 psyhical interfaces eth0; eth1; eth2

eth0 is used for my LAN network
eth1 throw bridge vmbr0 is used for my first VM (guest: Debian 6.0.8-amd64)
eth2 throw bridge vmbr1 is used for my second VM (guest: Ubuntu 10.04-server-amd64)

And the problem is... for about innactivity of each VM about 30 min. i have lost internet connection for my VM's
I can only connect via the web gui throw eth0 and using VNC from web GUI can connect to my VM's, once I start pinging any host from the guest machine, the Internet is presented immediately.
What could be wrong?

=====================================
For my first VM /etc/network/interfaces

auto eth0
iface eth0 inet static
address 212.142....
netmask 255.255....
gateway 212.142....

=====================================

=====================================
For my second VM /etc/network/interfaces

auto eth0
iface eth0 inet static
address 109.73.....
netmask 255.255....
gateway 109.73....

=====================================

Her's my /etc/network/interfaces for my HOST

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 192.168....
netmask 255.255.255.0
gateway 192.168....

auto eth1
iface eth1 manual

auto eth2
iface eth2 manual

auto vmbr0
iface vbmr0 inet manual
bridge_ports eth1
bridge_stp off
bridge_fd 0

auto vmbr1
iface vmbr1 inet manual
bridge_ports eth2
bridge_stp off
bridge_fd 0