Prusa i2 Z axis homing issue

Dear Friends,

I'm assembling a Prusa i2 (Marlin + RAMPS 1.4) and I'm having an issue with the Z axis homing.
The other axes (X, Y) are working just perfectly.

Here is the behaviour I'm observing: once the printer is started, Z axis is somewhere far from its home.
With manual commands from the Repetier, I can only move it high for a certain length (say, 10), and then I can move it back down of 10, no more than that.
If I try to press the Z Home, the stepper motors start to make a bad noise and do not move. If I quickly press and release the Z endstop, the motors start quietly moving down for a certain length. If I repeat these steps, that is, press Z Home and press/release Z endstop, if the Z axis reaches the endstop, then the motors stops.

Do you know what is the reason for such a behaviour?
And a possible solution for it?

Thanks a lot and ciao,

Flavio

Edited 1 time(s). Last edit at 05/30/2016 06:24AM by flavioc.

Solved: it was the default Z feedrate (set originally at 5) that, according to some posts, shall be decreased to 2.
With 2, the problem was still existing for me.
I just tried with 1, and that was!
Z homing is now working like a charm. Happy about that!