When the ign. module senses low voltage, it goes into a "stutter" or "limp" mode. Purpose being it's designed to allow you to get the plane back down on the ground without quitting completely. You'll only have about 25% power so make sure you're first approach is a good one!

Not an NiMh fan here either. Sounds like you have a dead cell in your pack - something NiMh is pretty well known for doing.