Some programs have reported to crash. It really depends on the particular CPU, mobo, software & temps and theres no definitive list.
Its not a new 8 series thing, its been out for a while. Id be wary of having it on. Again its not intel but a thing creatd by mobo manufacturers.
No, i dont think it will give you more FPS. The 8400 is a very good CPU. For better gaming performance look at your GPU.
What do you have? maybe sell it and get a better one.