Some games freeze my system completely

Maybe your computer is too slow and can't handle overload. That some games propably uses bunch of instructions that need compute power or you manually set more processor time to emulator and system dosen't have enough compute power to work properly.

This is completely wrong. A slow computer will just run slowly, that is all. No such thing as crash due to overload (assuming you don't run out of ram and have swap disabled I suppose)