It is your battery. You stated in your video you ran it around for about 5 min then it started and continues to get worse the more the battery gets lower. That 2000Mah nimh battery can't provide enough amps or for very long for your BL system. I have some nimh 3300 mah batteries that does the same thing with my BL systems the lower the voltage gets on them. They're just cheap batteries. They work fine for brushed motors but not with my BL stuff. Better batteries usually stop the slow driving cogging issue.

Whats your newer batteries? Try to get 4000mah+ in a nimh battery or go lipo(if possible) 3300mah+ with atleast a 25c