How does one go about troubleshooting VPN problems?

Hello all,

Our site is having intermittent problems with its VPN connection; sometimes
it works just fine and other times it will drop a connection after a few
seconds. This problem doesn't appear to be location based - that is, people
working remotely occasionally experience problems.

So far, all I've been able to determine is that our Win2k3 based VPN server
is working fine. I see the connection and authentication attempts from the
various users in the Windows event logs and there is nothing in the event
logs that would indicate an error with the VPN which would then lead to a
dropped connection. I've also gotten some of the users to do a tracert from
their local machine whenever they have connectivity problems. In some
cases, it is evident that a poor network connection on the user's behalf is
what is causing the problem (for example, one user had an average hop time
of a second and a half going through twenty one hops). However, other times
the user is going through no more than seven hops with an average turn
around time of less than 10 ms and is still getting kicked. How does one go
about troubleshooting this problem further?

Thanks.