Prusa i3 Z motor problem

[Alex_M]

It is a Prusa I3 With the 5mm Threaded rod. I have #define DEFAULT_AXIS_STEPS_PER_UNIT {78.7402,78.7402,200.0*8/3,760*1.1}// default steps per unit for Ultimaker
#define DEFAULT_MAX_FEEDRATE {500, 500, 5, 25}// (mm/sec)
#define DEFAULT_MAX_ACCELERATION {9000,9000,100,10000}// X, Y, Z, E maximum start speed foraccelerated moves. E default values are good for Skeinforge 40+, for olderversions raise them a lot.

[printbus]

It is a Prusa I3 With the 5mm Threaded rod. I have...
#define DEFAULT_MAX_FEEDRATE {500, 500, 5, 25}// (mm/sec)...

That value of 5 might be high, but like Mjolinor said, don't mess with the firmware yet. Find the setting for z-axis movement speed in the slicer and experiment with it. A number of factors come into play here, including the stepper driver current limit setting, the weight of the carriage that the z-motors would lift, acceleration, and moving up vs. moving down.