BM 3.8 SP3, VPN Client 3.8.9, NetWare 6.5 SP3, NetWare Client 4.9.1

All of my clients can connect using C2S, for some it's just like being on
site others report problems once connected.

I can reproduce this behaviour reliably:

Connection 1
Take a laptop with the appropriate clients, give it a valid external IP
address and plug it in the external switch into which the external network
card of our VPN server is connected (ie remove any outside ISPs).
GroupWise, file browsing and file opening, telnet clients and consoleOne
all work fine on the laptop.

Connection 2
Now take the same laptop, make it DHCP, plug it into our backup ISP
connection on an ADSL modem and the same list of apps work well.

Connection 3
Finally take the same laptop to another site (my house)plug it into a cable
modem and it will authenticate and carry out the NetWare login without any
problems. But, while I can see the top of the directory structure on a
NetWare server, as soon as I attempt to browse deeper, I'm told by Windows
explorer that the directory does not exist. I cannot start GroupWise with
the client as a standard install, even though I can ping the GroupWise
server. I can also ping the File server. Our telnet app will not connect,
but I can ping the server.

Has anyone seen this sort of behaviour before? Any suggestions?