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I have had fails for a lot of reasons, but this one is different. It happens when a perfect print is running, and the printer goes completely power-down off with no LEDs, fans, and black LCD. Fuses are ok, filament running smooth, layers going on clean, and the matterhacker screen remains up, although disconnected. I can restart by turning the power switch off at the printer and then back on. The printer lights up, but it can't talk to the computer until I disconnect at MatterHacker and reconnect. Then it thinks it's ready for a new print, and it extrudes immediately.

Random Observations:
I have reset max extruder temp to 300 deg (for E3d) in Repetier, but I am now running ABS at 220/65.
I also reset the extruder stepper power setting to 150 (from 165) because it tended to run hot. It still feels hot to the touch. The same crash has happened twice, once on either side of the reset.
The fan is 100%. The nipple is a .6. The layers are set at .4. Speed is dialed down to 20mm/s, but it seems to run faster. Speed settings seem to be wonky.

I am trying to print the new hotend mount I found on another part of this forum (tips & Tricks/"new E3D v6 mount?"), and the point of crash is half way up one of the three long slender vertical support structures that might involve some short extrusion/retraction steps. But I'm not getting clogs; I'm getting full crash.

Any clues appreciated

PS trying to recreate the crash tonight & I will see what the Gcode terminal has to say (wish it would copy paste).

I had this same thing happen when my power supply was failing. I don't know if there are other possible causes, but a new power supply solved it for me.

It may be a power issue. I had the same thing happen. The printer would suddenly shut down with no lights on the RAMBO board. I took the Z tower acrylic piece off and the side panels off. Then the printer works well. So my problem was power supply overheating.

I would check the following items.

1) Power connections to RAMBO board are tight and not loose or touching each other.
2) Power switch connections are tight and not loose and disconnecting.
3) Remove the back panels (around Z tower) and see if that works. This indicates overheating of the power supply.