Switching to NiMH from alkaline drops your battery pack voltage from 4.5v to 3.6v. I highly recommend getting a 4xAAA or 4xAA holder, and using 4 NiMH for a 4.8v pack. You're very close to the cutoff point for the NB, and it'll exhibit odd behavior as your batteries run down.

You can also tweak the settings on the SD card to adjust swing and clash sensitivity.