An ESC is creating a high power and high frequency PWM signal creating a magnetic field. Not directly in the 2.4GHz range but more in the range of internal data signals. Any high impedance data signal can still be influenced by external electro-magnetic fields.

Another thing I have heard. Do you have a 3 wire RC4 transponder put in the battery slot of the receiver? That seems to cause issues with a Sanwa receiver.