Hi Kronik,
as you confirmed that you have a USB-modem, is that the only connection type available, or are you able to connect it to your computer with an ordinary CAT5(e)/6 ethernet cable too.
With my modem I can use either USB or ethernet cable but the latter is the only truly working one. In "ordinary" use (using browser, file transfer etc.) you don't notice any difference between them but you really need an ethernet connection when playing any game online, otherwise you are going to meet problems sooner or later.
Test your internet connection sending larger packages to see how it can handle them (for example: ping -t- f -l 1472
www.altbierbude.de). If you lose packages often or even every now and then you can blame your internet connection. Otherwise the problem is an in-house problem.
So if you have a choice try ethernet cable.
Cheers,
Timo