Strangest Gmod Issue.

Okay, well this has got to be thee most strange incounter I have come across. I launch Garry’s Mod and as soon as I get in a server I start to freeze up and my ping/latency shoots up beyond playable conditions. Now as soon as I (if in windowed mode) click outside the game for a second, it will load everything. My graphics are not the issue, I can play this game no problem with the current graphics card I have because I can play normally in my own multiplayer server, this is only in other people’s multiplayer servers that I have this issue. And I also don’t have any network configurations to set up. The game is properly let through my firewall and I have a fantastic internet connection. Why on earth, or what on earth would cause my game to freeze up in such a way?