I have a fast 5MB connection.  Works GREAT most of the time.  I can download anything I want at almost 600kb/s, no probs, everything's lovely.

Except when it comes to games.  Basically, getting a server list in games messes up my connection, but JUST for games. 

I'll use CALL of Duty 4 as an example:

If I refresh the list of servers, pick one, then connect: it will connect (sometimes it will stick on one of the steps of connecting, usually "synchronizing"), the map will load, but once I get in it will show Connection INTERRUPTED and it doesn't recover.  In fact it doesn't recover until I restart my computer, and sometimes I have to move the ethernet cable to the other slot.  When I get in game it's actually normal for about 1 second.

Now, if I have a server on my favorites list in Xfire, and I go straight from the desktop to that server via Xfire, everything will be perfectly fine.  Excellent ping, no lag, etc.

It does this a myriad of other games too.  Counter-Strike SOURCE, Team Fortress 2, Crysis, etc.

Crysis is the worst one, since Xfire or no other service supports "one click join."  You have to go in game, log in with your account, then once it does that it will automatically get a server list which of course F's everything up.  The other games have the same behavior as Call of Duty 4: get list, connect, loads (or sometimes doesnt), and then connection interrupted in-game.

Also, once it's messed up, I can't get a full server list, or usually I can't get a full one in the first place.  Like CoD4 will list 20 random servers with pings all over the place, when I know for a fact there are at least 12,000.  Same thing with the Half-Life 2 engine games I mentioned, and for them it even happens when I try to get a server list out-of-game using Steam.

There are a couple games that aren't effected.  Company of Heores for one.  I don't recall F.E.A.R. Combat being effected either.  Even when the connection gets screwed up for the games, it's still perfectly fine for EVERYTHING else.  Surfing, downloading, what ever.  I'm typing and posting this response on a "messed up connection" after trying to play Crysis.

That's about all the information I can think of off the top of my head.

I have a NetGear CG814GCMR wireless modem (but I'm connected through ethernet).  Dynamic IP.