Sounds like the battery protection function of the ESC is kicking in. There's light, standard, and heavy discharge protection. I haven't check mine in a while but I seem to recall experiencing the same this problem and I think I changed mine to heavy discharge. I'd have to check to be sure.

Just make sure you time your flights so that you are only using 80% of you packs capacity.

I'll get easily 6 minutes on a 2200mAh pack and put in a little over 1700mAh.