I also had the same failure when I first test ran my engine. It seems that the spark plug wire was free to vibrate at the PC board and the 3 or 4 tiny wires holding the shield to the ground plane on the board broke due to this movement.

I took the cover off the box, wrapped some #24 buss wire around the shield and the holes in the PC board several times, soldered it securely, and made sure it wouldn't come loose again.

Then, I used a small plastic strap to hold the plug wire to the firewall and keep it from vibrating much.

So far, no problems...