There are a couple of things that could be:
1) Your Z-axis could be losing steps. If your Z-belt is too tight, it will take more force to turn the Z screw than it should. This will make it harder for the machine to lift the Z up than it is to lower the Z down, and it might occasionally miss steps. If your carve is starting on the bottom of the image and working toward the top, this would explain why the image is deeper near the top.
2) You may have a problem in the GCode sender or receiver. If the problem happens at nearly the same place in every image you cut I would check to see if something is interrupting the software, like a screen saver. kicking in.