I can’t say what the problem is with your algorithm, as I can’t see most of it. I assume you just have another logic slip up somewhere.
What I’ve done is put together another way of doing the same thing. More simple. See what you think.
I’ll put the pictures in there, but also upload the BP as a text file. All you have to do is open the txt file in notepad ( or similar ), select all, copy, and paste into a fresh BP. Then right click all the variables and choose ‘define variable’.
Basically it just stores the position of the original bounding box of the cube. On each iteration, it chooses a random point in that box ( X TravelMultiplier ) and moves to that point.
TravelMultiplier and AxisFilter should be visible in the editor. You can specify which axes the cube can travel on using AxisFilter. A value of 1,1,1 mean 3d, 0,0,1 means only travel on Z etc.
The ‘nasty’ bit of vector math on Destination is just to add the original position on the unused axes to the cube only travels in the planes specified.
Hope you find it interesting… 
[link text][3]