Hey. I could use some help. I have played TF2 on PC for a few months now, and never had any issues with lag, since my connection is good. Now all of a sudden, I'll join a server, play for about 15 minutes, and then I get lag so bad it is unplayable. It happens like clockwork. If I quit TF2, and start it up again, it's fine for a few minutes, and then lag again. 
 
It's really frustrating, since I know it's not my internet connection, and I can't think of any way to fix it. Help would be appreciated. Thanks.

Sorry don't know if I can help you sorry, I'd just say to check that nothing is downloading/updating while your playing.  Switch off the anti virus and itunes and make sure that steam isn't updating any games or downloading new ones.  alt-tab to steam to give it a quick check.  If that doesn't work just close everything that touches your internet besides Steam and TF2.  If that doesn't help then I don't know what to do. 
 
I'm having problems with TF2 crashing when servers switch maps lately, probably something to do with my new config files

You could try clearing the game cache of defragging.