Symptoms sound exactly like a fuel pump relay issue. I had a 1991 744 which had the problem of fuel suddenly cutting out. I would feel a loss of power, then see the RPM drop regardless of pedal position, and finally all the dash lights would come on as if I just turned the key to start the car (all this would happen randomly while driving). Loose solder joints and heat can cause contact points in the relay to work intermittiently or fail entirely. Unfortunately, I don't know much about the fuel relay system in the 850...yet...