NM disconnect after one hour with a wired network

[Charles Cultien <c cultien isae fr>]

Hi,
I have a very strange problem :
I'm connected with the wired network of my university and each time, 
after one hour, Network Manager disconnect himself and didn't reconnect 
automatically. So I have to reconnect by myself evry hour and this is 
boring !

I don't know why and how change this. A dmesg didn't show anything (on 
the moment it disconnect).

I'm using LInux Mint DEbian (so debian testing) with :
Linux kernel 2.6.32-5-686
GNOME 2.30.2
Network Manager 0.8.1
I'am am behind a proxy (automatically given by the script 
htpp://proxyconf) and so the gnome proxy is configured.

Thank you for helping and didn't hesitate asking for trying something or 
given more information.

Charles Cultien