MY 04 X5 4.4 had a similar problem. The vehicle would suddenly go into trans failsafe mode and I would limp home. It is a CPO so the dealer had it three times trying to figure out the problem. The only faults came from the accelerator pedal assembly. It was replaced under warranty only to have the fault return. The final visit resulted in the dealer finding a pin, that was almost broken, in a electrical connector going from the engine control module to the pedal assembly. This was the problem, long process but it is fixed now. Hope your issue is resolved. An X5 is not much fun in failsafe mode!