2001 X5 4.4 ltr 105000 miles.

I have chased this problem also. I can say what it isn't. Fuel pressures good @ 50 PSI, Crank position sensor good, Cam position sensors are good, no OBDII codes at all. Cleaned the battery cable conections @ the battery.

I most recently cleaned the power lug on the drivers side of the fire wall under the hood. It wasn't corroded but it looked glazed. I disassembled it and cleaned both sides. Knock on wood, it hasn't happened again since.