Unable to host WAN game in Garry's Mod - Anyone know what's missing?

[AciD]

So since I got this computer I've been having problems hosting Garry's Mod - Or any other 'regular' Source game at that. What I mean by the last part is that local hosting of Left 4 Dead servers work just fine, but that's the only one of its' kind.
Now, back to the topic:

I've forwarded my ports, and I know there's nothing wrong with them.
I've used sv_lan 0 and heartbeat, and I do not have "LAN game" set to 1 in the create multiplayer game screen.
I've tried giving out my external IP to friends, but they get "unable to connect after 4 attempts" error.
I've got no firewall at all (and hosting other games like Warcraft III works just fine), so it's NOT a general hosting or internet trafic problem.
When people try to connect through the Steam UI, it says it's hosted from "192.168.2.156" (which very much truly is my internal IP), and obviously can't connect.
The only difference between my old computer and this one in software is that I had XP on the old one and have 7 (used to have Vista, same problem) on this one. UAC is turned off.

All the measures I've taken are listed above. Please read them thorougly before responding, since that's a confirmation I've looked through the usual errors. Does anyone know any single part I might have missed? I'm starting to get really annoyed at this riddle and I'd love if someone knew the solution.

Thanks in advance.
//Aciaedius

 

[AnthraxXx]

If possible, try and plug the modem directly to your computer, as the internal IP is given from the Router which might be messing it up or it might be a DNS problem or something.

 

[AciD]

Afraid I'm not able to do that... There are several computers in the house, and the modem is far from mine. :/

But, I must ask... can an internal IP or DNS problem really specialize on only those games? As I mentioned, I can host pretty much anything except for the source games that go through the steam UI. A few of them are: Warcraft III, Left 4 Dead, regular web port 80 server... well, you get the point.
If you know what you're talking about I guess I won't be arguing, but just curious how it can make sense.

//Aciaedius

 

[moonrose]

i dont know if my problem is at all similar. but at lan partys i have one friend who hosts the steam games we play, TF2, CS:S, etc, when ever i try to join his game through the friends tab or through my friends list, i cant. i just have to wait for a another friend to get in and join through them. it cant be my comp, cause i reformatted and still same problem after a full reinstall of everything.
my point is, steam is messed up like that sometimes.
your probably better at asking here, steam help forums

maybe try reinstalling your steam stuff.