Truckracer pretty much covered it.

1st and foremost. Make absolutley certain that the plug cap is on properly. Some are incredibly tight to get on. make sure it has "snapped on" to the plug hex fully. This is a very common cause of ignition interference.

What is the connection between the throttle servo and the Throttle arm? Make sure you have a ball joint on the throttle arm - NEVER use a simple "Z" bend.

I can't see it clear enough but is the throttle linkage a solid wire inside a plastic covering or is it a genuine nyrod (no wire in the middle). If it is a wire running all the way from the throttle to the servo, you have a pretty good aerial there to bring interference back to the receiver.

If you can, move your ignition switch forward (and all the associated ignition related wiring) do so.

One other thing to check. Is your muffler tight? ( know this sounds weird but I actually saw an instance where a loose muffler contributed to an interference issue). My preference is to throw away the exhaust gasket and use Permatex Ultra Copper to seal the muffler onto the engine.

Lastly - as has already been mentioned - some ignition boxes just emit a lot of interference. If it ends up being that, Speak with RC Ignitions or CH Ignitions about retrofitting an RCExcel ignition.