There was a rather lengthy exchange of ideas in 2010 regarding the stop on the early style of Z-axis that uses a slot and formed bracket as a stop. Rather than repeating the whole exchange I will refer you to using Google's message base and go back in the date to messages posted around December 10-25. Search using the phrase "New Legs" which changed into "Z-axis pictures" which then changed to "Z-axis stop slipping - Early models" should bring up the topics, but Google's search is not working correctly at the moment. It's a huge mess of a thread to boil down, but here's a quick summary.
Replacing the plastic knob with either an adjustable hand knob ( http://www.mcmaster.com/#adjustable-handles) or a two handle knob so more torque can be applied to the screw. Add a washer between the plastic knob and the outside of the frame for a better grip in the slot.
Next add a screw to the bracket that holds the digital scale. This will prevent the bracket from lifting away from the plate that holds the router. Steve Hunt described this back in 2007.
"Couple of things may help you. The bracket that connects to the vernier is only held onto the chassis by a couple of small bolts, and only at the top of the bracket. This leaves the bottom of the bracket free to flex and will give false readings on the vernier. Securing the bottom of the bracket to the chassis with another bolt (have to drill and tap the chassis then a clearance hole in the bottom of the bracket ) will lessen the flexing."
Web Page added 2-115-2011