Web Service Calls Timeout over VPN

We have a VPN set up using Windows Server 2003 built-in remote access
capabilities. Our customers make a VPN connection, and then our
application makes a combination of SQL Server Database calls and Web
Service calls accross that VPN connection.

Occassionaly at one of our clients, the Web Service calls will start
timing out. The VPN client is connected and the database access calls
work fine. They can also ping the server successfully. Connections
from our officer work fine.

Any ideas why web service calls in particular would start failing? I
actually went to the client site yesterday, but by the time I got there
the problem had "magically" cleared up.

Once the VPN tunnel is established, what pieces can affect protocol
specific communications?

Thanks in advance for any help you can provide.

Karl