Z banding, indeterminate cause

So, lately my prints have developed a very unfortunate ribbed look, ruining the finish. see attachments. As the latest in a long line of issues with my printer, I began researching solutions and testing methods.
So far, none of the following have worked:
Changing the layer heights in case of rounding issues.
Keeping the z motors enabled between moves.
Loosening the coupling between the z motor axle and the threaded rod.
Constraining the top end of the threaded rod.
It's not z wobble, as I'm printing on a prusa i3 which has smooth guide rods. The effect is the same across all layer heights and types of filament. When printing the same object repeatedly, the patterns of banding are very similar. I'm also not quite sure it is the z axis and not the extruder, because when I printed a bracelet, which is much wider than the robots and calibration cubes, the banding was not consistent all the way around. That is, if it was a rounding issue, then the whole layer would be off, which it does in the smaller prints, but in the larger prints it goes in and out as it goes around.
Any suggestions? I've run out of ideas.

Edited 1 time(s). Last edit at 07/29/2014 12:40PM by dexterousfolder.

You're ruling out z wobble because you have smooth guide rods. In my experience, Z wobble isn't caused by the smooth rods - it's caused by the threaded rod - and the smooth rods don't always eliminate it.

It's difficult to see as you move from layer to layer, because it is only moving a small distance, but try this. Raise the x carriage way high - as high as you can go, and then hit the Home Z button. Watch the threaded rods carefully. Particularly at the couplers. If you see any kind of oscillation, this is what's causing the wobble, and you need to eliminate it. I had many problems because the flat ground into the motor shaft caused the coupler to not be perfectly centered. Even though I was using a high quality lead screw and 10 mm smooth rods, the wobbling was very apparent.

I just tried several different couplers, finally ending up with a set purchased from mitsumi, before my problem was solved.

Recently I purchased a set of motors with the lead screw integrated into the actual motor. There is no coupler and there should be no issue with coupler-caused wobble. I haven't started using them yet, but just based on examining them, I am expecting good results. BTW - these motor/screw combinations were purchased from robotdigg.

SOLVED. After some days of experimentation and many calibration cubes later, I found the source of the problem. The heat bed! As it maintained temperature, it was flexing up and down and changing the distance from the print to the nozzle. I turned it off, as I'm printing only in PLA at this point and it sticks just fine to blue tape, and now my prints are lovely and smooth. See attachment. Thanks for the suggestion, but it turned out to be entirely unrelated to the z axis! this also explains why the bands changed around larger objects...