Does the motor die completely or does it just refuse to accelerate?
If it does NOT die completely, my money would still be with one of the stick coils.
You have the twin spark motor. All 4 plugs fire at low rpm. Under hard acceleration or anything above around 80km/h, the lower plugs stop firing. Then only the top plugs fire. In the event of one stick coil being faulty, the motor will then only run on 1 cylinder.
To find out which cylinder is giving the problem, unplug either one of the stick coils. (Just the 1) Now take the bike for a ride. Accelerate hard. If there is no change in the symptom, you have unplugged the faulty one. If the motor dies completely, you have unplugged the good one. You now know which side to work with.
Check the little plugs where they plug into the stick coil. These are known to break quite easily. You can buy the plug with a short piece of the harness from BMW. (Part No: 83 30 0 402 341)
If the RHS coil is the one giving trouble, also check the RHS lower mounting screw for the ABS pump. I have had 2 bikes where that screw came out and chaffed the wiring harness (and the stick coil wiring)