Z-axis motors sometimes spinning in opposite direction

[behold]

Sometimes my z axis motors spin in opposite directions other times they do not. I checked the wiring and it is correct. I am using ramps 1.4 and arduino on the GEEETech prusa i3. It is almost as if when they receive resistance they reverse directions. Please help

[Mark]

1)if the wires are correct conect,it can't be working like this,please give me some more details .
2)tell me exactly your I3 type,I'll sent you the corresponding firmware,so you can try to reburn your firmware to fix this problem.