Simon,

I don’t recommend that you use a spark transmitter to test for noise susceptibility. Generally speaking, you will “kill” most CPU’s using a spark transmitter, which by the way, generates considerable RF energy. The RF is received by your circuit-board foils and sent to the ‘420 where it corrupts instruction execution. Always use ceramic capacitors in combination with tantalum capacitors for decoupling (4.7/0.1 uF is a good choice) in critical areas of your circuit board. I have used the ‘420 in several applications for computing floating-point numbers, and it works just fine. But I’m sure my designs would suffer the same fate as yours if I used a spark transmitter for testing.

Good luck

PS: You can also use an electronic-flash from a camera to test for EMI.