I'm seeing the same issue with mobs of Devidians. In this case I have them invisi-walled off from other enemy mobs to keep them from attacking each other, but they manage to do so anyway. Like erei1, I am also using tall invisi-walls, but that doesn't help.
I found my problem. It seems it came from some incompatibility between the broken forcefield and the invisible wall. I removed the invisible walls, just kept the forcefield, and it works fine. Finally.
@Pax, maybe the devidian have some special ability that let them go through invisible wall ? I know they can be "phased", maybe that's it ? Also, they fly. Try with something else, like I don't know, Cardassian, Klingons, just to see if it works.
Now I need to have some of my sanity back. I should ask to the leprechaun and the unicorn I saw before in the kitchen.