vpn disconnecting after a few minutes

Archived from groups: comp.dcom.vpn (More info?)

The vpn stays connected for only a few minutes at a time, and is also
slow (and erratic) in connecting.

Here's the details:

My machine:
- Windows XP
- connected via USB port to DSL modem
- internet connection sharing on
- Windows firewall OFF

My sister's machine:
- Windows 2000 Professional
- connected to my machine via crossover ethernet cable
- needs to connect to Nortel Contivity VPN; she can connect, sometimes
after more than one try, and more slowly than normal. This is the problem.

Why I'm setup this way:
I'm visiting my sister's house, where her machine is ordinarily the one
connected to the DSL modem. Her VPN works just fine this way,
connecting quickly and staying connected. However, as you would expect,
if I try to plug in to her ethernet port, the VPN disconnects, and she
needs it for working so this is unacceptable. So I thought her VPN
could tunnel through my machine instead, and put my machine between hers
and the DSL modem. Then, on my machine, I turned on ICS and disabled the
Windows firewall to get the VPN to work.

Any ideas on how to make this VPN setup more stable?