VPN users losing connection to exchange server

I have a VPN issue that has been plaguing one of my clients and wondered if you have ever dealt with it.  I have users connecting to a Windows 2003 server via MS VPN (PPTP).  They are having issues, where they ‘lose’ the connection.  When I say lose I mean they are still connected through VPN, but the connection to the Exchange server quits (‘Trying to reconnect to the Exchange server”).  By disconnecting the VPN and reestablishing it all seems to work for a while.  There doesn’t seem to be any consistent time frame that causes this (i.e. – sometimes is after 45 minutes, sometimes after a few hours, sometimes a few minutes, sometime not at all).  This is happening for all users (about 10 of them), they are located across the US and have different ISPs and equipment.