Sensored motors don't cog so it's not that. I'd say it's the drag kicking in if you are just easing on the throttle. Try and have zero (most do these days).
It could also be a faulty sensor wire. If you have it all connected and roll the car forward by hand, the end 3 led's should faintly glow in sequence. That's the ESC picking up the sensor positions. If that doesn't happen it may be a faulty sensor wire.