have the same problem on my dakar, specifically mentioned spark plugs and cables to the stealership, they refused to change the plugs saying mine is fine. It only happens at speed 120+ after about 40mins of riding. the techie said they could not simulate the fault, i asked weather he took it for a test ride, "Yep, i took it around the block"...after i told him to take the bike for a ride even filled the tank up for him...anywayz, gonna mention it at service time, and the better fix it, else im gonna start makin some noise.
The service manager even admitted to BMW supplying faulty spark plugs, but said mine is fne, and does not need swopping...