It seems I may be onto something with the notion of cooler temps being better.  Aside from the fact that no.9 seems just fine with it.

Cell no.10 has registered a 2degC rise in temp and a corresponding significant drop in output.  I've consequently lowered the heater temp as low as it will go, at best though that's only 2 or 3degC lower than where it was set before.  It may be time to try removing it from the heater, but I want to give it another night to see if we get a repeat of this behavior.