Please forgive the questions that I ask, but I don't know where you are in your R/C journey. Did you examine the armature, and make sure that it was very clean. Just new brushes with a dirty commutator (the part of the armature that the brushes run on) doesn't help.
Slash 4X4, Bone stock and trying to keep it that way.
PRO MATCH BATTERIES.....THANKS JEFF !!