lehner quark cogging

my g2r with a 1940/9 and quark 125b speed control, cogging real bad at stop if you push it it will run till you stop again then cog real bad. don't know if its the speed control or motor. we were running it pretty hard for about 15-20 min pretty much wide open, by the way it is a 5s 8000mah set up. any ideas were to start?? thanks in advance for the help.