sorry I'm new to the whole over clocking, what do you mean by "APM"
I remember it as "Advanced power management" or something like that.
It's a power-saving feature that causes the CPU to downclock. Unlike other power saving features that cause the CPU to downclock under light load, (Which is good) APM causes the CPU to downclock anytime it looks like TDP is going to be surpassed--even undr full load. (This effectively negates any OC)
Go into your BIOS and look for a feature called APM. If you can't find it, you probably have a BIOS version that doesn't allow you to disable it.
If you find it, disable it.