From the clue that increasing voltage seems to make it worse: Does the VRM maintain at least 0.400 volts VCCIN above Vcore? Perhaps a higher loadline comp (LLC)?
I would also suggest 'fully manual mode' but does the Hero BIOS have that?
Any adaptive voltages, EIST etc that vary with load could be slow to speed up as the workload varies. Check CPU power management, power saving throttling, power decay mode and the like.
At extreme loads where HT on/off makes a difference, DIGI+ Power controls like current limits, phase control and duty control should be set to the highest or 'extreme' mode. Those go for CPU and RAM power.
Let me know if any of these work since they're off the cuff suggestions.
Jeff