Sven, you need to eliminate one thing at a time really, firstly fuel supply.
can you hear the fuel pump run when you turn the key?
is the fuel pump wiring sound,
if so, you might want to fit a new fuel filter to eliminate fuel starvation.
Have you tried resetting the ECU by turning off and on the red switch, it may change the behaviour, depends what year the car is as the ECUs vary..
