Gerhard, I ran into the same problem as you. The "constructing molecules" shows up when the system is trying to launch the "Molecule" screen saver. I think it is some kind of bug related to the molecule screensaver.
I have already reported this bug (#72672) Meanwhile here is a thread at Ubuntuforums that has info how to change the screensaver to something other than Molecule. http://ubuntuforums.org/showthread.php?t=303894 -- System stops https://launchpad.net/bugs/72942 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
