I've had this issue for YEARS. I've never gotten it fixed across multiple computers and multiple modems (no router). It especially happens in Garry's mod more than it does in CS:S. I also have a server history issue. They don't show sometimes. I don't know why. I'm taking into account that it's an engine issue.

It has NOTHING to do with port forwarding. If you have anything open, close them - this is only for server hosting if you have a firewall enabled. You don't need them open to play games. You also only need your NAT configured when playing certain games. You COULD try that. Other than that, I have no idea. But I can assure you the ports do not need to be opened.