Regarding AHCI or required settings - changing the BIOS affects the OS drivers and lead to corruptions.
If your CPU is not performing correctly then it leaves as the root cause: CPU, MOBO, or PSU as the root cause.
I would reflash the BIOS first, next pull the CPU heatsink and replace the thermal compound -- meanwhile looking for paste on the CPU pins. Try another PSU on the off chance its not proving adequate power to the CPU.
After that you're left between the CPU and MOBO.