Many of the older 12 gal Cubes have been having that problem.
Balast issues. My LFS has 4 of their display Cubes open with MH lights hanging over them right now while the hoods have been sent to JBJ for repair.
Your best bet I am told ,would be to contact JBJ do see what they will do for you...
Well the cube normaly fits all the needed stuff in a single unit. So even thought it can be costly depending on where you are...it still might be cheaper than buying all the parts.
-Eco