Setting up a 'homeplug' wired connection

Hey,
I have a wireless 'N' belkin router, and I can provide the exact product code if needed, but anyway the problem is I connect via a ethernet home plug from the router to my xbox 360 and have around 4/5 wireless clients at max using router.

Anyway I get good download speeds on the xbox 360 and wirelessly on my computers but I keep getting 'Game disconnected' from like multiplayer games on xbox 360, I didn't use to get this on my previous router.

I was wondering what particular things could I do to make my connection more 'open' because I'm pretty sure it's a fault on my part considering this message pops up on FIFA10 on the xbox 360 the majority of the time and never did before.

Like I've got UPnP on, and I've tried setting a DMZ on the xbox's ip (but the ip changes, I think I have to use static ip which I did before and it messed up), I've tried opening ports but UPnP is on so...

Not quite sure what else I can do?

Thanks!