I’m assuming your specs are pretty okay, from the 200 FPS you suggested. One of the issues that I found when playing tf2, similar to yours, is I would be fine on the title screen, but drop to a few FPS in game. One reason might be your gpu is bottlenecking. If you have EVGA overclocker, I would change the setting to KBoost, which essentially stops the gpu from bottlenecking, and will help your experience a lot.

To put it simply, when I launch TF2 I start at around 160-200 fps, but it slowly crawls downwards. If I leave the game idle, it seems to drop to 15 FPS or lower. I'm just wondering if this is a hardware issue, windows issue, or game/config issue. 

I'm Having An Odd Performance Issue, Anyone Willing To Help Me Find The Source of It?