First, check for metal-to-metal contact. Always use nylon clevises with metal horns. You can use metal clevises with nylon horns. Make sure all nuts and bolts are tight. I that's not the problem, move the radio wiring around. Be sure to use a resistor plug. Keep all engine wiring in front of the firewall and all radio wiring behind it. Use a nylon rod for throttle control, not a metal wire. A last resort would be to shield the firewall.

Dr.1