It sounds like you need to delete a keyframe for the Z rotation at the point it flips. Set the box in the correct position and set another keyframe for it. Also, make sure you have the "Auto Key" feature turned off while doing this. It tends to add many more keyframes than you want for everything in the scene. You could also try creating your own motion path instead of using the preset path. It takes a little longer but you can at least be sure only the keyframes you make will be there. Good luck.