SSH timeouts from virtual Ubuntu workstation to bare metal servers in home network

[Hans Vos]

Hello,

I currently have the following setup.

- My PC (Windows 7) -> Virtual Box -> Ubuntu 12.04 desktop
- Server 1 (Ubuntu 12.04 server)
- Server 2 (Ubuntu 12.04 server)

The virtual Ubuntu desktop is using a "Bridged Adapter" configuration. 
When I setup a SSH connection from my Ubuntu desktop to one of my Ubuntu 
servers I get a timeout after a while, even when the session is not 
idle. It happens after a few minutes even when something is happening in 
the terminal window. I get the following error message:

"Timeout, server host.example.org not responding."

On my Ubuntu desktop I've already put the following line in my 
~/.ssh/config file but it doesn't help:

"ServerAliveInterval 5"

What might be the problem and how can I solve this?

Kind regards,

Hans Vos