Yeah I mentioned it's a UM2 in the topic, but didn't in the post.
I will try lowering the power. I'm familiar with Mxxx codes (I also have a Printrbot Simple Metal, an M500 + M501 is practically a daily operation for using it!), so I'll do that. I wasn't keeping track, but from memory, it would make sense that it is overheating. The prints I recall them failing on were fairly long (3+ hour) prints. There was also a bit of clicking, which initially I associated with the extruder, but never made much sense since the print wouldn't have issues. Might have been the z-stepper losing power for a brief moment!
BTW, M500 = save, M501 = recall saved settings (to confirm they saved correctly).