Hi

It was over a year ago that I had the same problem, I also asked the isp.
As you can think they dont had a problem and just said it must be the VPN
device (M0n0). The only solution for me was to lower the MTU (Windows 
machine).
Its really hard to track down such problems because an isp will not care 
about such
things.

regards
Andreas

ps:sorry for my bad English