The reason your CPU is throttling to a lower speed even though the temp is only mid-60s is because the CPU is drawing more wattage than your settings currently allow. It decreases speed so that it stays within the limit.
In the ONE menu of the BIOS are two settings: Power Limit 1 Value and Power Limit 2 Value. Change PL1V to 100 or so, and PL2V to 125 or so. Then save and exit. That should solve your issue.