Well all seemed well, but now, randomly, the Mega reboots for no apparent reason.
It's not under any load (no RGB leds are running), and when it's running off the bench power supply, it is pulling less than an amp.
But... randomly (sometimes 20+ minutes apart). it will just reboot. No idea what that would be 
No idea how to track that down.
This Mega has been very problematic. I think the time has come to bin it and re-install the entire rig with a Teensy 3.2 I have spare here.