How do I figure out where my computer is "bottlenecking"?

Specs:

• XFX HD 6950 1 gig ddr5

• AMD Phenom 9850 Quad

• 4 GB Ram

• Windows 7 64 bit

• terabyte hard drive

• decent mobo

• 600 watt power supply

I run the game on the lowest settings even with Chris' FPS config for highframes and I manage to get about 140ish when no one is in the server and I'm staring at a wall but as soon as the battle starts and I'm surrounded by bullets and explosions I often drop down to 60, 50, sometimes even 40 fps. This is a pretty nice rig I built in the last 6 months and, unless I'm wrong, I wouldn't expect this to happen on it. I also get random blue screens of death almost every other day but not while playing as much as while opening a new tab in chrome or when closing a program. Maybe memory?

Also I know some people would be praying for over 30 fps and think this is a kind of tf2 first world problem but when you spend 600+ dollars on a gaming computer and it's fluxuating between high and middle FPS when you're mid battle it can be quite distracting and annoying.

How can I pinpoint what is causing my computer to bottleneck? One small little note that could be an issue is that my GPU is very close to being to big for my case, it's gigantic and I didn't know that when I ordered it and it's a very tight fit inside my case. Could this be blocking airflow? It also fried a powersupply I had to replace it and I feel like some of the issues started some time around then but I could be wrong.