I don't know if your motherboard has a temp reading on your VRMs, and if someone else has an opinion I'd definitely recommend they give it.
IMO, this sounds like your motherboard is throttling your CPU to protect it's VRMs. What happens if you return it back to stock?
Also, as another thought. A 6300 turbo boosts to 4.1GHz. Reset your BIOS to default, turn on turbo boost, and see what happens with Prime 26.6 and HWmonitor.