So, as it turns out, the problem was with the mounting of the CPU cooler. I realized that to install that Zalman would require removing the motherboard (the Zalman was beefy enough to require an anchor behind the motherboard). I wasn't too enthusiastic about doing that, so I took the original (stock) CPU cooler off, cleaned off the thermal grease, re-applied a fresh batch of grease and put the stock cooler back on. The CPU temp now stays in the "normal" range regardless of load on the box, or the ASUS EPU-6 engine setting.
Thanks to all for your suggestions!