Thats quite odd, all your settings are fine.
Considering it wasn't prime stable, try upping the voltage a bit, may be causing the frequency drop.
Also check your voltages, does it drop under load and by how much? You have LLC, so your vcore may be dropping to around the 1.25/1.268 mark, which is very low for 4.2Ghz.
^ NOTE: doesn't mean its not possible to run at low voltages, but your board only has a 4+1 phase, so it could possibly be the VRMs can't provide a stable current to your chip.
Make sure your VRMs are cool.