extreme cogging!!!!

I to am haveing this same problem,on acceleration it coggs like a son of a gun, and it didnt do it until recently,i've checked all the bullet connectors and they fit very tight,i havennt pulled the cap off the motor to check the solder in the motor wires.ive tried to re-flash the firm ware,recalibrated the esc,when running my normal batteries(tenergy 8.4v 3800mah) it does it,when i put a 3s lipo it went away. anybody have any ideas?