You're experiencing glitching. Check to make sure that your frequency crystal is from the correct radio and receiver (same manufacturer and correct bands).

On the electric, if that's a brushed motor, you'll need a small capacitor on the motor to clean the RF around the chassis. Check some old threads on brushed motor capacitors.
It's easy to solder one on.

Finally try a glitch buster capacitor (much bigger) on the receiver. It can help. If your receiver only has 2 channels, just make a y-adapter.

I suggest you get a 2.4GHz radio though if you are sure you like this hobby. Even with PPM FM radio's (including ZPCM) I have from back 15-20yrs ago, it is not clean like my modern 2.4GHz radios.

GL.