This occured when Crown Technicians where on-site. So at least they believe me now :-)

With an external-computer connected to the forklift, the technician was able to figure out what was going wrong:

The controlling device that sends the data about the steering-wheel direction to the driving-wheels sends out spikes of wrong data. E.g. my steering wheel is at an angle of -90° but the controller tells my driving wheels it's at a -40° position. That's the reason my forklift busted out some crazy moves :p

We're getting a new forklift, 4200-series, because the technician didn't trust the 3200-series anymore (this happened to me on a replacement machine as well)

Atm I can't tell you exactly what's going wrong, but as soon as the technician knows more (he's having a meeting about this issue with the German main office) he'll mail me. And I'll make sure to post it for future reference.