occasionally X5 will not start, please diagnose

[cooa99]

Hi all, i need some help with my car.

I have a BMW X5 2005 (E53) 3.0Diesel

About 2 months ago, I had an instance after filling up at the petrol station and the car would not start. The battery was fine and cranking hence i knew its was not the battery.

Someone there suggested that it might be the immobilizer that i should lock and reopen teh car. This did not do the trick.

Anyway I sat there waiting for a tow truck (10min) and then the car started again. This sort of incidents occured a couple of times.

2 weeks ago, stopped at the supermarket and same thing happened, this time car did not start for well over 4hrs,
The Road assistance chap came and still no joy and i waited another 2hrs for tow truck to turn up (still trying to crank the ignition but no joy)
Car was towed back home and the chap said it might be key related hence that I should try my spare key, like magic that started the car up!.

So I started carrying both keys around pretty much using them Interchangeably as both are on my key holder now ...

Well yesterday on the way from the airport, I stopped to get my daughter some sandwich and the car wont start. I tried both keys and no luck, tried locking and unlocking the car no luck. I was on the phone ringing for a taxi when the car cranked back into live.

I drive daily to work (120miles round trip), so I'm only in the car at start & end of journey and no stops in between. All these incidents seem to have happened withing short stops.

please can you guys give me some indications as to where the problem might be?

Thanks very much

cooa99