Why does my PC crash when I try to tab down LoL but not more requiring games?

As I said i the title, when I try to tab down LoL my pc turns black(Slightly glowing black, that is). And about 10% of the times it actually works. I have to tab down sometimes when the games messes up, and with messes up it changes so that when i left click I do the fall back ping and i cant do anything else. Please help? I can tab down more requiring games as New Vegas and Tribes ascend and much more.