Ok, I can run any other Source game fine, but when I played the L4D2 Demo, I realized that I was getting frequent lag spikes. I then realized that it was because my CPU temps were getting above 60° Celsius.
I have an AMD Athlon 64 6400+ Dual Core CPU.
I idle at around 42° Celsius (which I realize is already high for just idling).
I tried reseating my heatsink (and re-applying thermal compound onto it, and yes I know how to properly apply it).
Any suggestions?