Its the stock adjuster... it does that in my car too...
Welcome to the world of cheap performance cars made by ford.
The correct way to do it is not to push down the clutch and let your foot slip off. That is kind of being hard on the already touchy adjuster.
Basically put your foot underneath the clutch and lift up until it stops... not hard but until it hits a stop. Then push down on the clutch very slowly and you will hear a click.... Then its adjusted and it should have no more play...
A firewall adjuster would help to replace the stock adjuster along with a clutch quadrant... check out
http://www.stangsuspension.com :-)