Every now and then my internet connection terminates

[Legato5589]

Grettings. I do have this problem for over a year now and just can't find out what the cause is.

First of all, this problem persists regardless of the operating system (WinXP SP1, SP2, SP3, Vista) and the computer hardware (both my laptop and my home computer have the same problem).
Only things that never changed are the ISP and the DSL-modem.

This is what happens:
I either play Lineage 2 or I am on an afk sellshop. Out of the blue my connection to the server is being terminated. The cause of this is that the manual internet dial up is disconnected.
I am not able to reconnect to the internet as long as the Lineage 2 client is still active. After I close the client I can connect to the internet without any problems.

This only happens in conjunction with Lineage 2. My internet connection won't disconnect while the Lineage 2 client is not running.
The problem does not occur while I play any other games online or just surf around/download stuff so it must have something to do with either GameGuard or the Lineage 2 client.

Help is appreciated. It is really annoying playing a Summoner and being disconnected every now and then.