Enemies permanently getting stuck ou ...

I got this game on steam with a buddy of mine, we got the complete DLC version, and I have two instances so far screencapped of times where orcs are getting stuck out of the map, or just bugging out, and not despawning, resulting in a forced exit. This is incredibly frustrating when we're playing endless and we dedicate time to playing this only to have it spontaneously halted with no solution, making it really hard to ever want to play this considering if you put time forward in it, there's no point because at any moment you could randomly be forced to restart from a bug.
I understand there's a despawn timer for normal mobs getting out of the map but we waited over 15 minutes on some of these and nothing happened.

http://oi47.tinypic.com/3307fit.jpg
This was in servant's quarters, there weren't even any traps down this close or near this I think... Maybe it was a teleporter though. The mob was invisible to us completely, still in his spawn I think (you can see the red dot on the minimap), we tried setting archers super close to see if we could do anything but no luck.

http://oi48.tinypic.com/m68i9.jpg
This mob was visible on my friend's screen but not mine, he was standing in place on his a little out of the corner, but was invulnerable, and if I attacked the spot the attack would stop where his invisible (to me) body would be but not do anything. We were just messing around with different ideas here and had steam traps in front and void walls with spring traps all along the wall down it, but he was smack dab in the middle at that curve. Maybe the steam trap pushing him up.

Also randomly on respawn sometimes I get a blackscreen, pressing escape brings up the menu fine and dandy, screen is back, but the moment I close the menu to go back to playing it comes back to blackscreen, and I can't do anything. I have a Radeon HD 7750 video card, maybe if you could look into that too, it would be great.