First make sure both of the batteries are good.
I rebuilt throttle assemblies for the V-12 engines before. The original grease and metallic dust (from regular wear) over time could turn to a hard paste and could cause problems. Also oxidation on the variable resistor plate could create dead spots.
I have also seen many failed DME (and EPROM inside of them) and throttle control modules and engine wiring harness problems. You need to find a good BMW shop who actually willing to work on your car. Even though the car is almost 20 years old the parts are still extremely expensive for it.
Technically this is one of the worst creation from BMW and most shops just simply refuse to work on it.