Are you using the native VPN client? I can only speak from experience with an enterprise grade Cisco VPN connection from work and I have zero problems with my connection with the Cisco VPN client. Although, if I really want to get work done I power up the Dell My friend has the same problem with his Mac and home VPN connections, not sure if he ever resolved it. I would make sure the software/firmware on your Mac is up to date and that your router is on the latest firmware as well.