Some possibilities:
Water in fuel? Check filter drain. Filter at rear underneath, unscrew drain plug. Drain to glass jar, water sinks to bottom.
Bad battery/charging system? Should be 9.6V while cranking. 14V + engine running.
Ford probably would have seen this though, unless intermittent.
Intermittent
PCM fault? Hard to diagnose. Expensive.
50km/h each time? Vibration/wiring related? Bad electrical connection? Visual check wires and connections in engine bay. Look for rubbing, fraying, clean and secure contacts.
Maybe crank/cam sensor intermittent fault? Replace.
Good luck.