Z-axis failing

My z-axis sometimes stops working, I call for the printer to move it and it works fine and then when I give another command and it doesn't move at all, I don't know what's causing it, it seems completely random, I can still hear a sound though as if the motors are trying to move but not something is holding them, and when that happens I turn off the printer and the threaded rods kinda rotates (like a quarter spin) the opposite way from where they were initially going.
Usually when it fails it stays not working for a while even after turning it off and on again, then it magically starts working again after a while, once or twice a "slap" on the printer unjammed the axis and made it work again.

Any ideas?

 

Can you try in repetier? Have you power cycled everything?

 

If by power cycling you mean turning the printer off and on again then yes.
I'll try it on repetier tonight

 

Maybe one of the wires is not pluged in to the board all the way. Flip the machine over and check that all wires are pluged in all the way. Be careful not to let the table or head slide around as you turn the machine over. The x and z axis can move around and break the z supports if you are not careful when turning the machine on its side.