It has to be something with how the post processor generates the gcode, or it's loosing steps during the cut. Charley in the Lithophane thread uses Aspire for his image engraving, so his PP may generate the gcode differently. The issue he had was with loosing steps and needed to adjust the pots for the steppers on his gshield to correct it.
Also open the PP in a text editor and check if these variables are set this way. With this setting, it will generate all axis moves on every line of gcode, even if an axis position has not changed from one incremental step to the next. Our image to gcode programs generate the gcode this way because we found it runs smoother with grbl. A stands for All and C stands for Change. Leave the feedrate to C since PVC does not have a variable feedrate feature like our PicEngrave Pro 5 has.
VAR LINE_NUMBER = [N|A|N|1.0]
VAR SPINDLE_SPEED = [S|A|S|1.0]
VAR FEED_RATE = [F|C|F|1.1]
VAR X_POSITION = [X|A|X|1.4]
VAR Y_POSITION = [Y|A|Y|1.4]
VAR Z_POSITION = [Z|A|Z|1.4]
VAR X_HOME_POSITION = [XH|A|X|1.4]
VAR Y_HOME_POSITION = [YH|A|Y|1.4]
VAR Z_HOME_POSITION = [ZH|A|Z|1.4]
In PicSender use the GRBl Z Code and Position Sent options.
For setting to Inch units in the startup block, type in $N0=G20 into the Do. Cmd window and select the Do Cmd. button. Also to set a default jog feedrate, type in $N1=F50 the same way. Set $13=1 (report inches, bool) this way in the grbl settings for inch units