mtu/mru problems with pptp

Hi!
My ISP provides Internet via a VPN so I generally followed this guide
(http://www.mreriksson.net/howtos/netbsd-pptp-client/) to install a pptp
client and I also set up a NAT to share the internet connection.

The default options provided by the guide worked well until I tried to
open www.google.com - the HTTP headers get properly recieved but the
rest of the page can't be loaded(due to corrupted packets).

After a few hours I found that a change in the mtu/mru options of pptp
made a difference - at mtu 1000 and mru 1000 google works perfectly but
guess which site stopped working properly - microsoft.com!

The host that has this problem runs NetBSD 3.0.1 but I tested it also on
3.1 and it still persisted. On windows and Linux, however pptp runs
properly.

Please, help, I ran out of ideas .. :)