The fan on the dummy load that consumes the required minimum current to keep the ATX power supply happy wasn’t starting up reliably. That’s not surprising: I connected it to 5 V rather than the rated 12 V, because the load heatsink needs just a whisper of air flow to stay barely above room temperature, so it’s barely turning over and has no spare torque at all.
It turns out the heatsink really doesn’t need any forced air flow, despite having the fins oriented crosswise. Without the fan, it stabilizes just above comfortable-to-the-touch, a bit hotter than I’d prefer.
While I had the hood up for the HBP rebuild, though, I swapped in another fan and the heatsink is now cool to the touch. I did clean that dust off the fins, too.
If this one also fails at +5 V, I’ll fiddle the wiring to put it across the +12 V and +5 V supplies, where it’ll see 7 V. That should improve its disposition…