The ESC in these are proprietary, and, I think, part
of an integarated ESC/reciever/servo.

How old are the ZZ racers?
It could be that the brushes are worn and as the motors heat up, there
is enough distortion to cause the brushes to arc. This could cause the RF.
Capacitors, if not already in place, could help to reduce this, but it may
be that the brushes or motors need replacing.