It took me some time, but I finally solved the problem. The most obvious was that the ECU misinterpreted a signal from a defective sensor. After studying the RAVE manual for several nights and carefully observing what went wrong and when, I came to the conclusion that it could only be the MAP sensor. It could be possible that it was just not defective enough to trigger a trouble code. I checked the ECU several times with a notebook and couldn't find any trouble codes, still the engine went in safe mode several times. It occured every time when the rpm was around 4000 and the pressure in the intake manifold was at its highest... To make a very long story short, I changed the MAP sensor (75€, 10min. work) and everything works perfect now.
Thanks anyway to you all for the tips!
Specially to David for his "Zen" approach, this helped me a lot: As a generalisation, an electronic fault is more often than not the ECU responding correctly to a faulty signal from a sensor.