BMW R 1200RT dies on long trips

Hello everyone. I am writing for a friend, Shuhei, who is not so proficient in English yet. He is working on it. He has a BMW R1200RT which starts up fine and runs like a dream, but on highway trips after he reaches about 1 hour or more of travelling very often will slow down and die on him. After he waits a short time, it will start back up again and repeat the process. He has taken it to BMW and they say they don't know what it is. My next door is into bikes (BMotor/Ducati) and is somewhat of a mechanic and his first impression is that it is the ignition coil and a heat problem as it seems to happen after the engine gets hot. Any ideas or experiences on this?
Thank you so much.
Michael