Shelving on my prints!

So ever since my mendelmax1.5 started printing some months back it has been doing this really annoying thing where it makes rings on the printed object. Here's what it looks like:

As you can see, the back of the gnomes feet have some shelves about every 10 layers, or i guess they could also be called ridges. And it's not only on one side of the object, the rings go all the way around so i don't think it's a wobbling axis. The same thing happened to yoda. This is his back:

Although this is green PLA it does the same thing with ABS. Also, the shelves are not always as apparent, but they are always there to some extent.
I have struggled with this problem for quite some time so any thoughts or ideas that may help me would be very much appreciated!

that's Z-wobble, I have top mounted motors on my Prusa Mendel, I don't know the wobble fix for bottom mounted Z motors.

Edited 1 time(s). Last edit at 01/18/2014 07:20PM by Dirty Steve.

Some possibilities:
[www.evernote.com]

its your layer height, what are your z axis screw pitches? and microstepping settings and what layer height are printing at?

Aduy nailed it. Your z axis pitch is not compatible with your layer height so the layer height rounds out periodically creating a compressed layer then it starts fresh and happens again. Z wobble would manifest itself on only some sides not all the way around.

Edited 1 time(s). Last edit at 01/20/2014 12:46AM by umdpru.

How are your motors for Z axis connected?

I saw this problem happen when the Z axis drive is set too high or resistance of the motors was too low. Basically what was happening
was the drive operated until it overheated and shut down, and as soon as the drive came back online it resumed operating until it overheated and the cycle repeats the entire build.