It's a very long time ago that I modded anything, but if it's true that 2v2 work fine while 3v3 and up cause trouble, then I would say that something makes the system struggle to run: 2v2 is fine, but 3v3 is just too much to handle. Which things are larger than the default settings?

I'm using broadband, but as Yuuki says: that's not a guarantee. It's possible that two broadbanders can't play each other at times/all. I experienced many problems when one had cable.