x5 3.0td auto sport starting problems HELP

[tonyfashlink]

having problems with my x5 starting..
starts 95% of the time starts excellent.. 5% of the time it wont start it will turn over fire for a few seconds and stop or just turn over, but not fire, however, if I crank the engine for about 10/15 seconds and keep doing this until the battery is flat and then get a jump start it starts straight away.
If I jump start the car with the battery still having some charge in it, it will not start just turns over, could it be a sensor that just does not open and sending a spike thur with the jump start opens the signal ?
Also put a new receiver ring on the ignition because someone mentioned that the key might not being read, but made no difference, they also mentioned that it might be the EWS
Many thanks and all info gratefully received

Regards

Tony