Intermitent ignition failure.

[Adie]

I have a 2002 X5 3Diesel. I have been having an intermitent ignition problem for the past 12 months +. The car turns over but will not start, the probelm is everytime its towed to the dealer they don't seem to be able to diagnose the problem. First time it was in was they replaced the glow plug relay. The second time the replaced an injector (depsite the fact the diagnostic still suggested it was running within normal parametres of -5 to +5). The third time they replaced a fuel pump. It is back with them again and of course the car is not failing! They want to replace all 5 injectors but have no support for that theroy. The other issue I have is that the emgine has been tuned/remapped by the previous owner and they keep falling back on that being an issue and the reason they don't know their @rse from their elbow. Any ideas on how to proceed. At this stage seriously thinking about ditching the thing! Please HELP