Computer Trouble

Okay, my hand-me down, middle-upper tier computer is acting up when it comes to games.
 
Whenever I run TF2, the game plays great. FPS exceeding 40 or so, no lag, etc. Then, after a few minutes, my computer slows down to a crawl. The framerate drops to below five, and everything else runs slower than shite. After a few minutes minimized, the effect stops, and I can play the game again. After the cycle repeats a few times, the computer just cuts out the middleman and freezes, unholy noise stutter and all.
 
This also happens when I play Doom, a game that can easily run on Windows 95. However, I can play it for much longer than TF2.
 
I’m convinced that this has nothing to do with my computer overheating, since I have six working fans and one ’tard fan, and the game can run fine at temperatures higher than what it first fudged up at.

Maybe too much stuff, not enough space?  
No, I’m not referencing that shitty commercial, I got too much stuff on my old PC, and it ruined the hell out of it, could barely play something like Minecraft on it. (even on lowest settings)

Sometimes my antivirus software likes to do things when I’m in the middle of a game.

@CrashSucksMariosaGOD
 
Did the game work fine for a period of time before effing up?

@SuperGex3: The Return of SuperGex  
Sometimes it would, sometimes it would just lag as soon as it started.  
(From Crash)

Specs? Peak temps?

Also, any recent driver updates or hardware changes?
 
The part you mentioned about temperatures makes me worry; having temperatures of 85c+ on frequent basis is never acceptable, and will usually lead to a very early death for the hardware in question.

@Namalsk
 
Windows 7 64 bit Home Premium  
AMD Athlon 64 X2 Dual Core Processor 6400+ (2 CPUs), ~3.2 GHz  
4096MB of RAM  
DirectX 11  
NVIDIA GeForce GTS 250
 
I updated the system a lot when I first got it in March or so. I’ve never gotten temperatures higher than 85, according to SpeedFan, although it’s gotten to 80 before.

@SuperGex3: The Return of SuperGex  
You should monitor your RAM usage while playing - this seems like a memory issue of some sort, if nothing is overheating.

@Namalsk  
How can I monitor RAM usage?

@SuperGex3: The Return of SuperGex  
Task manager should have a “performance” tab. Go to there, and in the bottom right it should say “Resource Monitor”. Click that, and it should open something like this.
 

 
It has no overlay, but all you should need to do is check it every now and then inbetween alt tabs to see if something is borrowing more and more over time.

@Namalsk  
Tanx.