Sonic Firewall Client Connection Quandry

Hi you lot,
I'm hoping you can help me. I work in a remote office with many pc's installed on a p2p network, we get on the net through a D-Link ADSL router and four of the pc's access our server at our main office using 'Remote Desktop'. The connection to the server is established through Sonic Firewall's vpn client software.
We have a random fault with one of the connections. Quite often one of the PCs will not get beyond the status 'connecting' when the vpn client software runs. The others all work fine. On the days that the computer in question does manage to connect..it appears to prevent the other pc's from connecting. A one or the others scenario.
It's completely puzzling me.. to date I've confirmed the integrity of the cabling, replaced the hubs in the building for switches and the D-link router that's now installed is a brand new replacement for an old draytek router that I initially suspected. So hopefully I've proved it's not the hardware here.

Ideas - most welcome.

James