Just put together my Nybble and I'm noticing a lot of under voltage issues. Any fast movements will trigger the under voltage alarm and stop Nybble. Often the Nyboard will reset itself also.
I've tweaked the servo voltage to about 6.1v which seems to be the best spot but Nybble still won't move far without triggering an under voltage alarm.
Using new batteries (SANYO 14500 800mAh) so I don't think that's the issue. I've even tried plugging Nybble into a 8v 3A DC supply from the wall with the same results.
As a last resort I've changed the Nybble.ino code to trigger under voltage at 640 instead of 740. This seems to reduce under voltage alarms a lot and lets Nybble walk for longer distances, but the Nyboard still resets itself a fair amount.