Steam randomly validating game files
Check to make sure that your computer meets the minimum specifications of the game you are playing.
If your computer does not meet those specifications, you will most likely run into freezing or crashing issues.
However, this may not always solve the problem – if the problem has already started occurring, the gameplay may be glitchy until you relaunch the game.
One trick you can use is setting Steam.exe’s CPU affinity instead – when you launch the game through Steam, it will inherit Steam’s CPU affinity and launch with the correct CPU affinity.
You’ll find this file in your Steam directory – C:\Program Files (x86)\Steam by default.
If none of these solutions helps, try Googling the problem – you may have run into a game-specific problem with a game-specific solution or just a more obscure Steam bug.