SSH connection problems

When using PuTTY or WinSCP on a windows client the connection dies after about 20 min. Now here is the catch, if I PuTTY to another Linux server and then use ssh to connect back to the problematic server the connection never breaks. The problematic server is a Dell PowerEdge 2900 with a Boardcom NetXtreme II NIC running Debian Linux.

I have upgraded the client software to the latest release. It is a bit better but still cuts the connection.

Any help would be greatly appreciated.