Computer screen turns off/flickering when I try to underclock cpu?

Lenovo z585
AMD A8-4500M
Radeon HD 7640G/7670M Dual Graphics
4GB RAM
Windows 8
I've tried GPU overclocking and works fine with AND Overdrive, but can't seem to underclock the CPU.
When I try, the screen goes blank or if I reduce by 1X, It starts flickering showing the login screen for less than a second and then going off again for a few seconds.
Help, I wan't to overclock (less than the Turbo Boost setting, but more than the ordinary setting) but can't unless I underclock it.