It could be a few things, but also questions need to be asked.
Is this happening in only 1 game that you play all the time, a few certain games, or all games. This is a big one. If it's in 1 or a few games it could be a few things.
Also how much ram do you have, and do you have multiple ram sticks? If so there could be an issue with one of the sticks. All the diagnostic crap doesn't always work. Trust me. I've used crap/software multiple times and it says it's ok, but the objects weren't ok. Once I removed/replaced a stick of ram, changed a hard drive, even a chip or a card it solved the problem. It's a machine and machines do some crazy machine crap.
Now if you have multiple sticks of ram, then you may want to try one stick at a time being run in the pc and play the game or games that are giving you issues, then try another stick. This can be a good way to see if it's the ram.
Also when doing one stick at a time make sure in the process manager crap you check if each ram runs at the same speed on it's own when you do that. If the speeds are different even if it's the same brand and type that could signal an issue or mean something is there.
Also sometimes turning down settings in games or certain settings can solve the problem. Turning shadows, light type, smoke complexity, etc etc etc can make it stop crashing.
Is your pc a pre built from a company, a person or did you build it yourself?
Sometimes parts don't always play nice together and most of the time might work fine when doing basic things, but as soon as you run programs that are very intense, they will crap out and show their ugly face in many ways.
I hope this helps. If it doesn't I'm sorry and I'm sorry for my crazy writing.