First, you could do what Walker said in the other thread and follow the Piezography instructions. Or learn to create custom profiles for your printer. I have tools and instructions that can help you do that (see the link below).
About your printer: not knowing the condition of the inks, or even which inks you're using, it is hard to say what your exact problem is and you won't know for sure unless you remove the variables that are obviously causing some of the problems.
The first step there would be to use a supported paper and quad curve. If you are still having posterization issues you can print an ink separation image and see if there is any obvious problem with positions being out of order or otherwise not printing correctly. From there you can determine if there is a mechanical problem that can be overcome by simply relinearizing an existing curve, having a custom one made for you from master curves, or creating your own curves from scratch.