VPN host to client - slow only to some VPN clients - How to fix

[HalScherer]

We have a Win NT 4.0 VPN host on a T1 line serving multiple VPN clients on the internet. One of the client PCs is getting intermitten huge delays.. ISP's are not willing to help diagnose the problem. They say if I can identify the server causing the problem they would look into it. But how do I do that. Trace routes to the PPP IP address only shows 2 hops. Using this IP address, the MAX MTU without errors is 510. Max MTU for other clients is normally above 1300.

Are there any tools that can be used to help me determine which server on a route is having problems with this VPN traffic.