I can't for the life of my figure this one out.

I have about 5 NPCs on screen that want to kill me. They all know I'm there.

If I walk up to them, they attack.

If I walk away from them, they have their weapon drawn and keep looking at me. I never "exit" combat. The never follow me to attack or run after me. They stay where they are. If I walk up to them, they attack.

Archers follow this; they don't move. They shoot at me from any range, however. They do not move closer.

These NPCs are essentially in combat with the player but never want to move from their spots.

I do have pathgrids.

If your pathgrids are good, then I would surmise that you have too many NPC's in the area. There is a limit to the number of AI the computer can control.

But, I really suspect its a pathgrid problem.

7 Total NPCs in the area. The pathgrids were generated - this is in a large worldspace.

EDIT: The NPCs also walk around fine when in their wander AI packages - if I'm not around/not in combat with them.

What do you mean by 'generated'?
Did you verify they are actually good pathgrids and setup correctly in the area with lines between them?

I mean the path grids were...generated. World -> Pathgrid generation. The behavior is the same as it was before there were any pathgrids at all.

I am placing a creature in the fray to see how it behaves.
- the creature behaves like the NPCs.

Also, one of the 7 NPCs is an ally with a follow player package, and he behaves just like the enemy NPCs. Just stares them down as soon as they detect each other and stops following me.

Sounds like a path grid problem to me. That is how they react when there is no valid path grid.

Make sure your using a clean save game.