Connection to external server via vpn constantly drops

We often connect to a server in another facility via vpn.  This works well from all external sites except the comcast serviced office.  Here the vpn connects and then drops every 30 to 40 seconds.  Frequency of drop is increased when we open Outlook 2003 on a windows 7 pc.  Do not have this phenomena from other sites nor with pc running outlook 2010 on windows 7.