3.3 is your nominal voltage. you can drain them down to about 2.5 per cell, but I would not go any lower than that. Always store then at 3.2 to 3.3 volts though if you are putting them away for a prolonged period of time.
And it is more than likely voltage sag that is causing that. If the pack is older, it will be worse than a new pack due to IR or Internal Resistance.