Z-ax motor blocked

Prusa i3.
Motherboard: Megatronics. + Marlin.

When I ask a Z-ax movement of 100 mm, it goes only 80 mm.
If I increase the steps per unit for the Z-ax I get problems when I put a number higher then 6800 steps per unit.
It goes perfectly up, but when I ask to go back to zero position, after 5 mm. movement the z-motors block making a big humming noise.

I cannot find any root-cause. Some-one familair with the problem?

Awaiting your suggestions,

w k r Henk

* MOVEMENT SETTINGS
*/

#define HOMING_FEEDRATE {50*60, 50*60, 4*60, 0} // set the homing speeds (mm/min)

// default settings

#define DEFAULT_AXIS_STEPS_PER_UNIT {92.5*2,92.5*2,6800.00,100*1*1.5} // default steps per unit for Ultimaker
#define DEFAULT_MAX_FEEDRATE {300, 300, 5, 25} // (mm/sec)
#define DEFAULT_MAX_ACCELERATION {3000,3000,100,10000} // X, Y, Z, E maximum start speed for accelerated moves. E default values are good for Skeinforge 40+, for older versions raise them a lot.

#define DEFAULT_ACCELERATION 3000 // X, Y, Z and E acceleration in mm/s^2 for printing moves
#define DEFAULT_RETRACT_ACCELERATION 3000 // E acceleration in mm/s^2 for retracts
#define DEFAULT_TRAVEL_ACCELERATION 3000 // X, Y, Z acceleration in mm/s^2 for travel (non printing) moves