Trying to find that max overclock can take some time. You might seem perfectly stable in a stress test, but not a video game. That does happen, but the fact remains if the crashing goes away without the overclock then it's not stable.
I encountered this pretty recently with my CPU, I was able to pass an hour of Intel burn test, but GTA V was still crashing.
I fixed it by adding a touch more voltage. In my case, 1.26v was able to get by on a stress test but 1.28v is what GTA V needed.
Sure, I could have just gone straight to 1.4v and it would have been stable from the get-go, but finding the right amount of voltage for an overclock you intend to use 24/7 is worth the effort.