Maybe the cause is not temperature [in all cases]. The experiment I'm running suggests a different root cause.
The machine I was using had 4GB of RAM. I have hooked up a different machine with the same OS, but 16GB or RAM. The ambient temperature is 81F (~27C), and after 2 hours, the print has not halted.
I suspect the culprit might have been Garbage Collection kicking in on the small machine, interrupting communications. I'll provide an addendum when the print completes, successfully or not.
[ 2017-08-13T12:15:-- Addendum: 20 hours into an 81 hour (est) job, and it is still printing. It is looking like there was something about the smaller (memory) computer that the uArm didn't like. ]
[ 2017-08-15T12:08:-- Addendum: My print ran to completion, and I have started another one without mishap.
Pass: OS X 10.10.5, 2.5 GHz Intel Core i5, 16 GB
Fail: OS X 10.12.6, 2.66 GHz Intel Core 2 Duo, 4 GB ]