VPN problem

I am having a problem with just one PC logging into our VPN.

We are running SBS 2003, and all of the machines are XP Pro SP2.

On one particular machine, the VPN link fails randomly for no reason. If you
click reconnect, it connects again straight away.

Something else that may be relevant, is it only seems to occur when there is
load over the VPN. By this, I mean that I can have the VPN connected for as
long as I want and not use it, and it stays up. When I start doing things
over the VPN, within between 2 and maybe 8 or 10 minutes I will get a message
saying "Link to XXX failed, reconnect pending".

I have searched everywhere for any information. The Machine in question has
all its updates, virus protection and firewalls. I have tried turning the
firewall completely off.

Any ideas???