D-Link DIR-655 Wireless Router acting strange

Okey, so here's the deal.

I have a 16.000kbps ADSL2+ broadband,
and a D-Link DIR-655 Wireless Router connected to it.

Currently I have a laptop, a stationary computer, and my xbox 360 connected
to it. All wireless.

Everything is fine when I have only one connected at a time,
but when I connect my stationary and my xbox 360 at the same time, it seems
like I get a crash somewhere now and then, and I cant figure out what it is.

When I logon COD4 to play, I get 'connection interrupted' every.. ~15-30min,
wich causes me to go offline.

When I download anything on my stationary, it seems like the internet
connection 'goes down', or 'hangs' for ~20secs.
That would be the same problem on both systems (x360 and PC).

I have tried to lock an IP to their MAC-Adresses, so an IP-Conflict could
not be possible.

I have UPNP enabled, and all ports etc.
Still this is happening.

Anyone got any ideas?
Would be appriciated alot