The pattern suggests a large oscillation which would indicate that your gantry is flexing. You might want to try lowering your acceleration ramp and your instantaneous start speed. Since you are starting and stopping in very short segments you are constantly accelerating and decelrating which upsets your gantry and causes it to oscillate. Machines without much rigidity and mass are very susceptible to flexing and vibrations.
As a test, plunge into a sacrificial piece of material and allow it to come to a complete stop at that depth. Then route a straight line from there for a good distance, say 20 inches. Take a look at the cut quality from start to finish of the line. You might notice that the oscillations or chattering is most pronounced at the very beginning of the cut and will diminish as it goes along the line. You may even notice some curviness at the beginning of the line which means the machine is flexing under the cutting forces.
For a sign building I would recommend you buy or build a more rigid machine to obtain cleaner cuts.
Gil
