Here we go.
No Lissajous, no frequency comparisons.
In a test made long time ago, drawing those symbols ("L" letters if you like), I got some on the leftmost position, notoriously distorted.
Symbols are drawn as in normal text lines from left to right, from top to bottom, one "character" at time (built spot after spot).
Knowing that, you will see that the distorted ones are preceded by a a character on the righmost position in the previous line. Otherwise, no distortion.
I asumed that the beam was arriving late for the first spot in the 1st char of the new line (left bottom spot in the character). In other words, code seemed to be faster than the beam. I still feel it is true.
I recall having solved this, brute force style, applying a 19-NOPs delay (16MHz PLL clock 18F micro) somewhere in the code. I believed that, in that way I was actually "waiting for the beam". Sole thing I can say, the symbols came OK after that.
Since I decided to discard the previous one before reworking anything, I want to know what to look at.
Gentlemen...?