In the case of the Fuji 50IE, I have located the trouble after extensive troubleshooting. The last thing I did which showed the problem was to wrap the spark plug wire with aluminum foil, not touching either end of the wire to ground. Engine ran fine but still had some jitter from the control surfaces. Upon clipping the foil to engine ground, the engine stopped! With the foil connected to ground, the engine would not start. Took off the ground and all was well. I decided that the plug wire is leaking badly and am returning the EI module and associated wires to Hobbico for analysis.
I'll let you know what they respond.