problem with vpn

[Eric Tanguy]

I'm not sure this problem is related to fedora but ...
I use a vpn connection to connect from home to my university. This
connection uses a java software client.
the connection is established so i try to make a nslookup something and
the system answers well. I retry the same nslookup command 1 minute
later and the system answer connection timeout.
I can't understand where the problem come from. The connection seems to
be established for a very quick time and after that all is down.
The same connection worked fine few weeks before so maybe it's related
to an update ?
If i reboot the same machine on winxp the vpn connection works like a
charm ...
Someone could point to me in a direction ?
Thanks

Eric