Basically, what I understand is the heavy weight of your Z-axis.. your motors have no holding torque to move it up.. You can do
6 things IMHO:
1- Check out all motor wiring and power supply, check wether power supply output voltage and current are the same what is recommended ..
2- Install a counter weight system at z-axis like i did in following thread
http://www.cnczone.com/forums/showth...t=39588&page=2
3- Decrease the acceleration and Velocity values in your CAM and Controller software
4- Decrease the weight of z-axis assembly
5-Upgrade to Higher torque steppers/servos..
6- last but not least, grease the linear rails and check wether any dust is accumulated in lead screw...May be Mechanical problem!!!
Best of luck
Regards
Khalid