Try to run each cube individually, write down the optimal settings for each die on that cube.  I would consider mine "stable" if they ran for a couple of hours without error. 
Once you have each cube dialed in, try running running them together, adding another cube each time.  First two cubes , then three cubes, etc.
When problems start happening, you'll have a much better idea of what's causing it.

If you decide to keep the rig, REPASTE EVERYTHING.