Thanks for the feedback y´all! Did some further testing based upon this, and noticed that the game not only produces pixeltrash on some fast systems, but also hangs on 040 and 060-cpu. The former bug I can´t reproduce so far, but I think it might have to do with me not coding a proper waitblit before feeding the blitter coprocessor. Thought that would not be needed if code runs from chipmem. Maybe a wrong assumption. Can easily be solved.
@Adrian Browne: You need an lha-unarchiver to run the demo. Final game will boot from CD-ROM or Disk.