on/off connection to office vpn, any ideas?

We have a home based employee that has a on/off problem connecting with our office vpn. Other employees connect fine, however with this one homeworker about twice/three times a week, she just gets the Error 721 message when trying to connect. No matter what we try we cannot get her to connect until about 90 minutes later when suddenly a connection can be made. This is always the case - about 90 minutes. As I don't know too much about this, I am hoping if I give more info then somebody can direct me as to how to investigate the problem. For the remainder of her working week she can connect fine.

Our company has a single SBS 2003 server. We allow access to home workers to a shared drive on the server through vpn. This employee has a Windows  XP pc, and to set up the connection she has used the wizard

- Control Panel - Network Connections, create new connection.
- Connect to My Network at my workplace
- VPN
- Company name called ourcompany_connection
- IP address - our external IP address
- Finish

For the connection part we have her using her username/password. This is fine for most of the week but can go down as mentioned above. We have also set her up a vpn connection with an admin username and password. When it goes down on her pc neither of the two connections will work whether its her username/password of that of an admin. We tried rebooting etc, but still cannot connect. Then baout 90 mins later, can connect.

She uses Mcafee Security Centre. We tried turning off the firewall that comes with that product but that makes no difference when she cannot connect. She uses a Netgear ADSl DG834 router. She does not have any other programs that I am aware that could block a connection. She does have AD-aware.

So what I should be I be looking into? Is there any log on Exchange server which could provide a clue or is this looking in the wrong place?

Any advice appreciated.

thanks