Today I had this problem and I didn't stop looking to fix it when finally, I did it!
My problem experience
I have played Portal 2 70 minutes in total before this problem, when I tried starting it again after a few minutes and sometimes barely one, the game crashed, "Portal2.exe has stopped working."
How did I fix it?:1.
Go to this directory (in my PC at least it's this): C:\Program files (x86)\Steam\SteamApps\Common\Portal 2\2
. In there you will find the Portal 2 application "portal2.exe", what you have to do is remove it.3.
After you removed it, try to run Portal 2 (it will say the application file is missing).4.
Now verify the cache (Library -> Portal 2 -> Properties -> Local Files).5
. If it worked, repeat this always you have the same problem, and enjoy the game.
IMPORTANT: Keep in mind that there are lots of bugs, and this may not fix yours, however it's worth a try.
PS: Excuse any grammar mistake I had, English isn't my native language.
PPS: If you had the game running, keep in mind that the cake is a lie.