The AI has a tendency to clumping up into a small hole made of usually gibs, and just staying there, not moving except for a few, on my cyborg zombie map.

Anybody have an idea as to what is causing this and how to fix it?
Oh, an they are set on AI_GOTO and targeted at the player brain.

That would be the retardness of the pathfinding. Either give them diggers or do some dynamic pathfinding of your own. Maybe shoot down obstacle rays every 50 pixels and set the Goto target about 30 pixels up from the place of collision. You could have a list of the resulting Vectors and have the zombies travel along them. Then update the whole path every 5 seconds or so.

Is it that place right in front of the guard tower? Because I notice zombies pile up around there.