I take it you've tried all the normal methods of solving interference problems:
1. Rearranging components.
2. Twisting wires.
3. Rerouting the antenna.
4. Installing capacitors on the motor (brushed motor only).
5. Making sure all solder joints are solid and shiny.

I've also heard that wrapping stuff in tin foil might help, but I'm skeptical. When I read about someone trying it, they always say it doesn't work.

The only thing left is to start changing out components.