[mpg200]

When under full load (6.89A I assume this is average and not peak) try and meaure each individual cell voltage.
This could give you an indication of if it's a battery issue or not. They should all be around the same 3.7V nominal. I had a similar problem with a helicopter.
A 3S pack would measure 4.2V per cell fully charged. But as soon as I gave it a load the middle cell would collapse to 2.6V. This would drop the pack voltage to 11V.
The heli would work with this but not for long, as the good cells would drop to 3.4 volts then the Lipo cut off would activate. Of course running at 2.6V on the middle cell soon killed it so it went to the big recycling center in the sky.