BMW Repair: Start proplem, airflow sensor, temperature sensors

Question
Hi.
I will try to explain to problem with my 96 740iL.
It takes up to 20 seconds to start. After turning the engine of it will start right up. If waiting more then 10 minutes the trouble starts again. We have changed fuel pump, pressure regulator, manifold gasket, injectors, battery, oil separator,crankshaft positioning sensor.
We have checked the fuel pressure, it stays constant even after 20 minutes. We are on a dead end.
Can somebody help?

Thank you so much  

Answer
Those are the first ones I would have checked.  

I wonder if it might be a cold start vs. a warm start.  If so, could be plugs or a temperature sensor going to the engine control or a problem with the airflow sensor.  (Though I would suspect runnability problems if it were the airflow sensor)

I'd try plugs, then the temperature sensors. I believe you have two temperature sensors on that car, both near the water pump.  Check if the wiring/connectors are good to both.