problem cogging with neu and xcelorin

hy,

i have a eight buggy with the losi brushless kit with the new xcelorin controless and neu 1515/2d/f
i use a zippy 4s 5000mA and a 15 pinion

i have a problem : there is a little cogging when i push a little the throttle

here are my settings



where is the problem

sorry for my english but i m french ...

thx so much for the future answers .