Pretty sure I'm having the same issue, and it appears to be demand related. With regards to steam, check to see if you have thumbnail caching on, if it is, turn it off. Steams resource demands exponentially increase, if thumbnail caching isn't turned off.
For me, I can't run WoW and a Skype video session concurrently, my system just doesn't like it. Though, my PC is quite old at this point, so I'm pretty sure I'm reaching it's limits. However, when I stopped doing that, my system stopped randomly restarting.
I'm not sure where to find that.
While testing portal 2 it does seem to be demand / heat related. After alt-tabbing to the game it runs fine for 30 seconds, then starts to get slower and slower, staying in the same spot. Frame rate drops from 30fps to less than 10 while turning. FPS is fine again directly after alt-tabbing. At least I know the fan is still working as I can clearly hear it speed up and down. Maybe I'm expecting too much of it, it's just a 2.2ghz i3 with integrated graphics.
The weird thing is it still doesn't go over 50% cpu utilization according to the task manager with multi core rendering enabled. It might melt though if it did lol.
It's not crashing now while stress testing it ugh.