As can be seen in the image the X-Axis is not really stable. The blocks should be 10x10x10 mm, and the top and bottom are perfect. The waving does not appear in the Y-direction, it's only in the direction of the bed movement.
Each block was printed at different speed, from medium to dead-slow. I limited the acceleration and in the last block I also limited the non-printing speed so the bed would never experience a higher acceleration than with the printing itself. But the wave effect remains. And it's also remarkably repeatable so it looks more like a synchronisation error somewhere. Does anyone has a clue, or experienced a similar effect ?
This is the issue of your z-axis rather than x- or y-axis because printing speed doesn't have any influence on the results.
- your threded rods are bent or
- nuts on these rods have too high clearance or
- nuts on these rods have eccentricity or
- couplings (between motor and threded rods) are bent or
- these couplings are badly fitted
All above suggestions concern z-axis.
 because it's PrintBot its z-axis has the following disadvantage
enter image description here">