XBOX Live and Linksys WRT54G router probs

Kinda not the right group, maybe, but since buying the WRT54G router I've
had nothing buy problems with XBOX Live.

The router is deeemed as certified by MS, yet I get problems with voice and
the problem that some people cant join a game im playing, or I cant join a
room if one particuar friend is in that room. If he leaves, I can join !!!

Ive tried everything - Dynamic, Static, in the DMZ zone, ports forwarded -
pretty much everything else I can think of.

I have the latest firmware installed.

Any other ideas ??

Cheers, Simon