The basic plastic printerbot frequently produces bands on the sides of the prints which match the z-threaded rod pitch. This is most obvious on prints using .2mm or thinner slices.
In order to find the source of the problem I made custom parts to lock the bearings and rods in place. I also made an external bracing.
The source of the banding appears to be a rocking in the z-motors.
This causes uneven z-lift which in turn causes some layers to be squashed and buldge on the sides.
This is easiely fixed with a few wedges but the other parts may be of some use for people wanting a more rigid bot or to re-purpose for something else.

Also the z-motors are .5mm too far out on my bot. This is an error in the orginal CAD files.
I have a corrected but untest version here. thingiverse.com/thing:26542
I will be publishing more modified parts when I find time.
I have done the CAD for a modified base with extra motor support but this is overkill for most people.
thingiverse.com/thing:28557