Try what rothers posted:

Code:

ping googlemail.google.com -f -l

Keep reducing the packetsize until you no longer get the response

Code:

Packet needs to be fragmented but DF set.

I'm not sure if I remember the theory correctly here, but you may need to set your router mtu to no more than 28 more than your good packet size.

It takes a while for me to be able to do such tests on the router, as I am in a flatshare, and should only mess around with the router when no-one else is using the internet.

Anyways, I did the ping test recently and what I got was:
If <packetsize> its greater than 1472 I get "Packet needs to be fragmented but DF set."
If its below that, but greater than 1364, I get a "Request Timed out"
And if it is 1364 or less, the ping completes successfully...

I don't understand why there is this packetsize range where I get a "Request Timed out", or what that means actually.
Also, I tried setting the ppp_mss setting to (1364-28 ), and now the problems seem to have cleared. If I set it back to 1400 for example, the problems are back.

Although this seems to have solved the problem, does anyone have any idea what is happening here ?

daiphoenix wrote: Although this seems to have solved the problem, does anyone have any idea what is happening here ?

No!

FWIW, if 1364 works, you may be able to increase mss to 1392 (1364 + 28), I think. Having said that, if it works now, you may as well leave it alone.