I had the same problem recently. I had a Netgear 602 Wireless Router/Firewall between my DSL line and my local LAN. I could connect up to a Sonciwall VPN and stay connected for days. But when I would try to connect to another client's VPN via the Win2k3 VPN Server, it would last for exactly 2min, 43 seconds...disconnect and then reconnect. I checked every setting on the MS VPN side as well as my RAS connection setup...but was unable to solve it.
The Netgear started locking up so I took an old Pentium II pc and installed SmoothWall (Linux based firewall/router http://www.smoothwall.org/) and removed the Netgear. Since that change, I can now stay connected to the MS VPN for 24hours or longer.
So my suggestion is to first check with what Firewall/Router you have locally. Try swapping it with another product or build your own and see what happens. Smoothwall even has a floppy-disk boot version so that you can take a pc and boot from a floppy without wiping the hard disk. Would be easy for you to test using this method if you have a spare pc with two network adapters in it.
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