'Sine wave' deviation in X-Axis on moving bed printer.
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.

I bet:


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.

[edit] because it's PrintBot its z-axis has the following disadvantage