i'm surprised nobody said this earlier but, your batteries are only rated for 50a constant and 75a burst. with low ratings like that, it is bound to puff and your esc is in danger as well. if you want to get some batteries that have better ratings without spending $200, you should look into the hobbycity lipos.
at the minimum, i recommend this battery:
http://www.hobbycity.com/hobbycity/s..._15C_Lipo_Pack
the battery is mislabeled and is actually rated for 20c constant, 30c burst. it uses the same cells as the zippy flightmax packs. 100a and 150a are much better than 50a and 75a. you should notice the power difference immediately.
for only $40, that pack is a steal!