All Steam games start by working fine, then slow down to a stuttery crawl

I have Steam installed along with CS:GO, HL2, and Payday 2, and this happens in all of the games: when I start a game, the game seems to run fine at over 60 FPS. Then, the game starts to stutter, to the point where there are severe FPS drops (2 FPS).

I am running a Core i5-2520M (dual-core hyper-threaded) with HD 3000 graphics. In Windows, I get lower FPS (30+) on my games, but I have never seen frame drops like the ones I'm getting on Ubuntu. In fact, everything always ran smoothly. I don't know whether this is an issue with the drivers, or it's an issue with thermals.

The Additional Drivers tab shows:

The output of top shows:

What can be done to fix this?

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