Welcome to the world of single-rail diesels!
Without some knowledge of the antennas in use, and how they're mounting, it is anyone's guess. I suspect the engine cutoff is due to common mode (poor antenna mounting scheme??).
I have to assume you all haven't done any bonding (hood, exhaust system, etc.), or other noise abatement procedures??
I worked with a G4 a couple of years ago, and got his Hilux down to about S2 without the NB on. I suspect this is about as low as you'll get, based on the injections system used.
So, how about some more information? Photos?
Alan Applegate, KØBG