Is it the ESC or the Motor??

TA05
Trinity 27T motor and 19T motor
LRP AI ESC

At full speed, I can hear the motor would run... and then no power... and then run.... and then no power.  Seems better if I press the motor brush a little, but then it only better for a while, and then do it again??  

What is it?  The both motors are quite new, and the brushes are still very good.  Need to clean up?  Need WD40?  Need to change the springs on the motors?  

better take a pic of the comm.  is it black?
check the slot. found anything stuck there?
what do u mean by no power? not turn?

problem solved with motor cleaner and this:  
Trinity Commutator Cleaning Pen

It works great!!!