Will not start,good battery,spark and fuel.
 

My 3ltr petrol 2001 X5 recently developed a slight misfire on very rare occasions.On friday it started fine ran for about 1 mile before a misfire and loss of engine power(not electrics)and refuses to restart.my local BMW specialist is struggling to isolate the fault after ensuring there is a spark and fuel is present it still fails to start. any ideas? I should add that it has a LPG conversion fitted about 2 years old fully serviced etc,but the fault appears before the lpg cuts in anyway.
thanks
Anthony