I am working on a zombie game. The zombies all chase after the player but he can easily avoid them by running around because the zombies are always chasing after the players current position. There is another problem that all the zombies essentially get clumped together (if there is no collision between them) they will actually move to the same position. Which can be solved somewhat by flocking code, giving you some separation but you still have the never reach scenario.
Do you think if I employed some sort of pacman AI it would be better. So some zombies would chase after him (the red guy), some zombies would predict the position where he is heading and cut him off
http://www.valvesoftware.com/publications/2009/ai_systems_of_l4d_mike_booth.pdf
That might be of some assistance.
As a frequent Left player, I can assure that some zombies will indeed seek a spot ahead of you based on your path, in order to cut you off. 
I could see that working pretty well in your game, too.
But the real challenge in Left comes from the fact that zombies can pretty much reach you at any spot. They are very good at climbing over obstacles, and spawning ahead and behind of you, etc.
Compare this to most other zombie games, where the zombie seeking ends up exactly as you describe; Clumps of slow-moving, easy to avoid targets.
Well that is really impressive and I might just need to give LFD another play! I am working on something alot more primitive.
The solution I think is to simply have more zombies and make the area smaller.
http://www.gamedev.net/community/forums/topic.asp?topic_id=563154
I play a lot of Killing Floor too, and it takes zombie kiting to an art. Most levels are designed so that you either hole up somewhere and kill them as they amble in, or you just run and kite.
There are some enemy types thrown in particular to offset both tactics, such as Fleshpounders, which will rush towards you if enraged while destroying all smaller zombie types in the way. Sirens have an area-of-effect scream attack that can kill a tightly-clumped group (ie: as you’d find in a hole-up defense position), and so on.
Maybe you could use a collider, that’s only collides with another zombie, to make them stay away each other’s patch.