There is a least one known issue that can produce this:

http://support.microsoft.com/default.aspx?scid=kb;[LN];Q315523

Here are some comments from an MS engineer on it:

http://groups.google.ch/groups?hl=en...40cpmsftngxa06

There are several Google groups hits on this, by the way:

http://groups.google.com/groups?hl=e...&sa=N&tab= wg

Simon

"Simon" <spamplin@hss.com> wrote in message
news:bdpe27$k71$1@hercules.btinternet.com...
> I have an environment that has 18 separate SQL server machines. When
> connected to the LAN I can connect to each and expand the databases tab no
> problem. When I try the same over a VPN I find that three of my servers
will
> connect but on expanding the databases I get a long pause then 'No items'.
> I've checked around and found some references to the same problem (SQL
> server mag site) but no resolutions. I'm running XP pro and my SQL is sp3.
> The three servers in question have only TCP and named pipes in the net
lib,
> they are all on Win2K one is on sp3, one on sp2 and one is 'out of the
box'.
> The VPN has ports 1433 and 1434 udp and tcp allowed.
>
> Does anyone have any suggestions as to what is causing this, its got to be
> something simple but I've been looking at this so long now I cant see the
> wood for the trees.
>
> Help appreciated.
>
>