How are you charging this battery, with a wall charger or a smart charger?
The quality of your battery is probably very poor. If you wanted to try a better NiMH battery before you invest in a lipo, charger, and MOSFET/low voltage detector, head over to www.cheapbatterypacks.com
and get a better battery and charger. Or you can get AVd and you might be able to find one in the classifieds.