When it dies on the road, it typically just jerks the car, goes down to 0 RPMs and then goes right back to where it was before.
This bit of info might change everything or it might be steering us the same direction.
Depending on the make/model of the car, there are numerous ways it can be getting the signal for the tachometer. The engine isn't actually going to 0 RPM (you'd have to use the starter if it did) but instead whatever is signalling the tach suddenly quit doing so and it wouldn't be at all unusual if it's the same thing that tells your ignition system (when) to fire the plugs.
I'm doubtful that oil-soaked plug wires could do this, though.
If we knew the make/model/engine, we could get a better idea whether the tach going to zero means "The coil isn't firing anymore" or "Whatever's telling the coil to fire isn't doing so at the moment."
This could be as simple as the crankshaft position sensor (CPS), which is a high-failure item. I've encountered it quite a bit especially on Jeep vehicles.
