Could be the driver overhead issue on Nvidia GPUs that hardware unboxed is talking about. GeForce GPUs apparently tax the cpu more. Have you tried playing at higher settings? As this would make the game more GPU bound and less CPU straining
Read laptop reviews to get a clearer picture of what you want to buy. I wouldn't worry about stability though because laptops come pre-configured. The OEM's test them and are 99% sure of their stability.
After you've made sure that the above are ok. Reduce the variables. Try using one RAM slot 8GB and see if it works well. Don't try to overclock anything leave everything at stock settings