We talk about this problem here:

http://www.nessoft.com/kb/29

The VPN changing thing is a bit of a clue, but it's hard to say a clue for *what*. Maybe the VPN software itself is causing this (we've seen this).

If you have another computer behind that same router and modem, try that and see if it changes things - as that might help isolate the problem to the hardware or the computer/software itself.

- Pete