TF2 Graphics Troubles

[arfy4]

So, I'm having this weird problem on the mac that I play TF2 on where on some occasions I will launch the game and it'll start off with when I am connecting to the server I can still see the servers window and when I do connect I can see through the walls. With some walls it's like they don't exist, and others I can only see players through them. This makes playing very difficult sometimes. Can anyone tell me if it's a problem with the graphics card or just the game itself? I will try to post pictures tomorrow. If it's a graphics card problem, the entire computer is still under warranty, so that can be fixed, and if it's a game problem, can anyone tell me what I could do as a solution?