Wheezy as internet gateway: lost connections

Hello list,

I have a small home network and using a Wheezy installation as the
gateway here. Since the gateway is running wheezy I'm experiencing
dropped (tcp) connections.
I observer this behaviour with both another Wheezy installation
(desktop) and with a Windows 7 machine. The problem didn't occur with
the old server (Etch).

The network setup is quite simple, the gateway establishes a PPPoE
connection with my ISP as ppp0 (actual interface is eth1). Another
network card (eth0) has a static address on the LAN.

The machine has a load of firewall rules for incoming and outgoing
connections, but the important ones here are the following:
iptables -P INPUT DROP
iptables -P OUTPUT DROP
iptables -P FORWARD DROP

iptables -A FORWARD -i $INTIF -o $EXTIF -j ACCEPT
iptables -A FORWARD -i $EXTIF -o $INTIF -j ACCEPT
iptables -t nat -A POSTROUTING -o $EXTIF -s $INTLAN -j MASQUERADE

Last line in my firewall script enables ipv4 forwarding:
echo 1 > /proc/sys/net/ipv4/ip_forward

What I see is various websites that don't load (such as slashdot.org or
hotmail.com) and services such as msn, IRC still works fine.

I did some research and it might have something to do with the MTU value
of the connection as explained here:
http://www.netheaven.com/pmtu.html

I tested this by setting a lower MTU value on my local (desktop)
connection, this seemed to work, same for the server. However it doesn't
have much effect today.

Does anyone have any idea on how to solve this? What changed between
Etch and wheezy with regard to MTU and/or packet fragmentation?

Regards,
Steven

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